Warhammer 40,000, contrary to 'popular belief' isn't all about lining up your army and then proceeding to run at your enemy and/or shoot, shoot and shoot some more. There are tactics involved and this is most often conveyed during the movement phase. Why? That's when you have as complete control as possible over your army. Your units can move up to X" with various bonuses or reductions depending upon how far they move (i.e. how many weapons a tank can shoot) and only terrain is going to restrict this. This is why random movement is bad (hi Fantasy) and why terrain can be so important (randomness is now entered into the equation). With this premise set up, being able to disrupt your opponent and specifically their movement or deployment, is a very valuable tool. Let's take a look.
When building a balanced list for competitive play there are a couple things your army obviously needs. Mobility, ability to deal with tanks and infantry in spades, durability (whether through numbers or good stats), a way of applying your firepower effectively, etc. Basically you need to be able to kill the enemy and restrict their ability to kill you (or simply not be wiped off the table quickly). Sounds simple right? Some of the factors that don't often make it into lists but which can often make a huge difference are units which not only do the above, but do it in a way where the opponent has to specifically cater for them. This is disruption.
What do I mean by this? Basically anything that is non-standard deployment or has the ability to affect an opponent's movement. Now, every unit in every army has this option to an extent. A Rhino for example is just as capable as blocking something as a Tau Piranha or Space Marine Land Speeder but has different opportunity costs, is unable to deep strike into play and has no melta weapon attach to its hull. We'll ignore the difference in speed here though that is an obvious advantage for the Speeder/Piranha as well. Whilst multiple Rhinos, Razorbacks, Dark Eldar Raiders, Devilfish, Chimeras, Immolators, etc. are great at restricting your opponent's movement and are an important part of controlling the battlefield, they are not disruption units. You're taking them for some other purpose (i.e. transportation) and whilst they certainly help in disrupting your opponent's ability to operate effectively, true disruption units go beyond this.
Specifically, by going beyond the simple expedient of blocking movement, disruption units force your opponent to potentially change their moves or deployment. Why? Because of alternative deployments such as deep-striking and outflanking. When units can deploy in a non-standard way, even scouting and infiltrating do this, your opponent has to consider them. For example, whilst outflanking a whole army is really unreliable, a couple of units capable in combat or with meltaguns? You need to block off your short table edge(s) or those units will come on to maximum efficiency. Against deep-strikers or scouters - again, with melta weapons? You need to layer your army with bubble-wrap units so those weapons won't get the best shots when they land, etc.
Now this might not seem as great as I've made it out to be at first glance. After all, if the opponent has an easy answer to these non-standard deployments, are they really that useful? Absolutely. Whilst their effectiveness may be diminished by your opponent changing their movement or deployment, you are influencing what they do. This is huge as you are forcing them to react to what you do rather than allowing them to enable their own battleplan.
What's important to understand about these units is they are not all about disruption. If they are, you need to look long and hard at them and consider are they really worth it. What happens when their disruption is not needed? Are they cheap enough and not taking up a valuable FoC slot they can still find a use in their army or are they a one-trick-pony? You should take that pony and leave it at home if it's the case. A unit which has disruption built into its role which can still tackle the battlefield 'regularly,' is going to get far more use.
For example, Mawlocs are great at disrupting parking lots or bunched up infantry but aside from, are a very expensive and pretty weak MC. Wolf Scouts on the other hand are going to fiddle with any opponent who has a backfield, whether it's a tank or infantry. Wolf Scouts bring excellent anti-tank capabilities through melta weapons and are good enough in combat to beat backfield units with a few upgrades. Against armies with no real backfield however and Wolf Scouts are less useful but they can still deploy with the main army and are an extra meltagun and combat potential. Conversely a MM/HF Speeder is going to do everything the Scouts do in terms of dealing with tanks and infantry but also has the ability to block movement of both tanks and infantry due to its speed and deep-striking ability. This is the ultimate in disruption and whilst it is less powerful in terms of actually changing your opponent's plans compared to the Wolf Scouts, the Speeder will always fit well onto the battlefield.
Conclusion
Disruption is great. It's not necessary and by no means not having any doesn't make your list bad, but they are great at changing the way your opponent operates. Whether it's during their deployment or how they move in later turns, impacting what your opponent does outside of simply avoiding your threat ranges and deployed guns is a nice asset. It doesn't always have to even be used but the potential threat it poses can force your opponent to think about it and react accordingly. Don't go overboard though and certainly look at the units you are taking to disrupt your opponent. Do they have other uses if they aren't disrupting your opponent (or can't)? Are they expensive or taking up valuable FoC slots? If so, consider whether or not you really need them. Remember, disruption is great but not necessary and it needs to fit into your army list smoothly.
