One of the major changes with 6th edition is Pre-measuring. There is only one thing you need to take from this:
- there are zero excuses for not being in the correct range band in a given turn
So, pre-measuring is both a blessing and a curse. It's a good thing because you know exactly where everything is in relation to everything else. It's a curse because you can get caught up in measuring exactly everything. This can be a time-waster and if you're not used to eyeballing distances, can cut corners and drag out time again. Let's break down what I mean here.
For anyone learning the game under 6th edition, pre-measuring is taking away what was previously a skill which could seperate players. The more important skill was knowing WHERE one's models needed to be but being able to put them, to a certain degree of accuracy, at that location was a skill itself. It generally affected the lower to middle players at tournaments more as it's something generally built up through experience or some crazy math brain. It wasn't a differentiating skill at higher levels though (generally speaking) as most players were on roughly equal footing with being able to determine distances on a tabletop. I'd prefer for this skill to stay in the game as it did differentiate between certain skill levels but I understand the opinion of many in eliminating it by bringing in pre-measuring.
Except it's not really eliminated. If one player can eyeball distances well but the other cannot, let's say they've only learned the game with pre-measuring in place, the former is going to be able to make decisions on the table-top better and quicker because the pre-measuring is a safety net for them. It's something which should be used absolutely - it's there afterall but rather than something the first player is relying on, it's backing up what they're already doing. By being able to do this mentally you're also denying your opponent insight into what you're thinking. If you measure EVERYTHING they know everything as well and if you're measuring certain units in relation to other units, etc. they could potentially derive what you're doing and start countering it in their mind.
Basically by being able to do judge distances without needing to use pre-measuring you can be thinking on your feet more often and then confirm it with pre-measuring next turn. It's an aide in decision making as you should already know what options you have and what you want to do but by using pre-measuring you can see which is the most likely. A common example is checking charge ranges - if one unit is 13" away and through terrain and another unit is 14" away but in the open, by getting the exact measurements you help your decision making process which started well before you started measuring.
Beyond this getting model placement perfect is now very possible with pre-measuring. Armies which rely on keeping the opponent at arm's length for as long as possible such as the Eldar variants, 24" based weaponry armies such as Asscan Razorbacks, Grey Knights, etc. can use their mobility and positioning to sit on that range band perfectly for as long as possible (whilst not foregoing board control). Ensuring characters or special weapons in units are protected during charges but can still make it into combat by making sure they're a set amount of inches back from the front can be done with pre-measuring. Knowing the exact distance a tank needs to move during a tank shock to get the exact placement you want can be done with pre-measurement, etc. These are all things which can be done with pre-measuring beyond aiding decision making but again, it's something which should be adding to your decision making rather than starting it.
For newer players or players who struggled with distance judgement before, I would encourage not using pre-measuring as much as one might like in their early games. Try to develop the judgement skills and movement skills so pre-measuring can become that safety net and tool to aide your decision making. Or rather than using it as much as you can, or just plain regularly, do all your movements and then check with pre-measuring (assuming this is okay with your opponent of course!).
Ultimately, in the best case scenario, you're using pre-measuring to get things EXACTLY right - you know the basic dimensions of the board and where things are, etc. but you're using pre-measuring to make sure your models are placed where you think they are, double checking ranges to determine your movement options, etc. It's a safety net or back-up to confirm what you've already formulated in your mind which should speed the game up when applied correctly. If you use it all the time to determine everything, you'll certainly pick up distances over time but rather than a safety net it becomes a necessary option which could slow down the learning process overall.