5th Edition has, as part of its design philosophy, encouraged and enabled mobility in armies. No more the stand 'n shoot gunlines of 4E; with cover being so relevant and most missions requiring you to deal with multiple objectives, you cannot afford to simply sit in one place for most of the game doing nothing. But more than that, the 5E codices have made it possible for armies to effectively mobilize themselves in a variety of ways, with each new codex having not only mechanized builds but also foot-mobile builds.
But over and over again I see the same mistake being made: people are mistaking deployment options- that is, flexibility of deployment- for mobility. These aren't the same thing at all, and confusing them can be fatal to your battle plan.
What do I mean by deployment flexibility? Well, by far the most common offenders here are Deep Strike armies, especially Drop Pods and Chaos Daemons. However, other forms exist, such as Outflanking, Infiltrating, and other special deployments. In all cases, the unifying factor is the ability to deploy outside of the normal restrictions of the battle type and the inability to exceed the usual bounds of movement. The latter is an important distinction- BA Jumper armies may deploy by Deep Strike, but they retain the ability to move quickly even afterwards. Tyranids and Grey Knights, to contrast, are no faster than many armies but retain the ability to fire to more or less full effect on the move, giving them acceptable (if not amazing) mobility.
Flexible deployment is a good thing; it can open up a wide variety of options that many armies may not have; it allows you to concentrate your forces and your firepower to selectively wipe out portions of the enemy army; it can shorten the clock you put an opponent on when aiming for melee; and it can present the other general with a complex set of options for what to expect, allowing them to make mistakes that you can take advantage of. Do not misunderstand me: all of these things are good for your army and can be critical advantages when used right.
However, they are not mobility. They do not allow you to outmaneuver the enemy nor to respond to their maneuvering, which is critical. It doesn't really matter where you start against an army of Space Elfs, because most of them have the ability to redeploy almost at will if they find the positioning unsuited to their tastes. Suddenly, all of your drop troops are stranded 20"+ away from them with little hope of ever getting into range and my point is brought harshly to the forefront of your mind. Similarly, though perhaps not quite so glaring, is the case of a mechanized list outflanking you to negate whatever cover you were hiding behind, or making last-turn objective grabs on parts of the board you can't/didn't get to.
Mobility gives movement options over the course of the game. It allows your battle plan to be fluid, changing as circumstances require and shifting to take advantage of the enemy's failures or to plug holes in its own lines. It is, in short, a continuous benefit, one that applies over the whole course of the game. Some armies may lose their mobility due to transports being destroyed, etc, but this is the result of enemy action, not a natural consequence of the way their army functions.
Flexibility of deployment is different- it is a one-shot affair. This is the critical difference, because if that one boost isn't enough, the enemy's mobility will overwhelm you. This is one of the reasons that all-Drop Pod armies, Daemons, Stealer Shock, etc, are subpar armies in general- lacking any real mobility in most cases, they can't deal with a mobile opponent. If the enemy outmaneuvers them, they are stuck where they are. If they wish to try for a distant objective, they must dedicate a unit to it early on (which is easily dealt with by a canny opponent and detracts from the army's overall firepower available). They are flexible at the start of the game, but lose that flexibility (barring other mobility or options) later on.
