Back in the olden days, being pinned was a bad thing. You didn't get to act next turn at all, which was super-sucky. This, amongst other things, was what made Fearless such an awesome ability- even Ld10 fails a non-ignorable portion of the time, so being flat-up immune is very nice.
Nowadays, though, it's not so bad- you get that dandy +1 to your cover saves, making you a lot harder to kill most of the time. In fact, it's so useful that they gave you the option to pin yourself pretty much any time you want to under the name "Going to Ground," and it's a highly underused part of the game, not just for expendable guys like Guardsmen and Termagants but even Marines as well.
For those of you still living in the dim mists of prehistory, cover saves are good. If you're a wimpy guy, your cover save may be the only real save you have; if you're a tough dude, a cover save is like an invulnerable that you didn't have to pay points for. And unless you're a tank or monstrous creature, cover saves are EVERYWHERE, so you're going to have them pretty much all the time as long as you play with reasonable terrain coverage.
Sometimes though, that ubiquitous 4+ just isn't enough, and that's where going to ground comes in. It's a devil's deal that says "Hey, buddy, I'll help you out, I just need a little something in return." You pretty much get the Stealth USR for free*. (*Free means you have to give up your turn and also maybe your immortal soul.) What's not to like about that?
Well, obviously the "not getting to act" part. What's the point of staying alive if you can't do anything? Well, it's only a turn, for one thing. The turn after you'll be back on your feet and ready to hassle the opponent's plans again- and heck, a lot of time your simple presence will be enough to disrupt their plans and they'll HAVE to keep shooting at you until the unit is dead, so going to ground makes them dedicate a lot more guns to it than they would otherwise have to. Always force the enemy to expend maximum effort when dealing your units, because 40K, like all wargames, is all about resource management. Time, distance, shooting, kill points, VP, whatever- the whole game can be measured in different resources and how you trade them.
This doesn't mean you should always go to ground, though- shooting is, as often as not, designed to eliminate one of your units from the battle, and only rarely will a single round of firing do so. If the enemy can shoot one LasPlas at your squad and then direct his other fire elsewhere, you're not really hurting him. You need to think carefully about the costs vs. benefits of not acting for a turn in return for increased survivability and whether the opponent will be able to ignore/bypass the unit once it has ducked its head down.
So when should you go to ground?
1. When the unit is going to die anyways.
If your unit of three Marines has twelve wounds incoming, you already know that they aren't going to make it through, so why not hit the dirt? Sure, you will probably still die, but there's always the chance. Likewise if your ten Termagants have twenty shots on them- going to ground significantly increase the chance that you'll live to see another turn.
2. Kill Point games
This one seems like it should be obvious, but I still don't see a lot of people do it. In KP games, every death is relevant, so even keeping a useless squad alive is better than not; sometimes complete obscurement isn't available and all you can do is crouch behind some trees and roll that 3+ every turn, forcing your opponent to spend extra shooting.
3. When "just being" is the unit's purpose.
Not long ago at a tournament, I had my Synapse web stretched rather thin, as I was down to two squads of Tyranid Warriors and I needed to keep my Gaunts, Hive Guard, and Harpies under control. (There had been some weird shooting going on, suffice to say.) However, staring down a bunch of Dark Lances, I was not optimistic about my chances of lasting 2-3 more turns with them, until I realized that I didn't actually need my Warriors to do anything so much as I just needed them to be there. So every time shots came my way, my Warriors would hit the dirt and, through some small miracles, weathered all of the shooting- one lucky individual in particular personally took fire from two Ravagers from three turns straight before he finally succumbed. Other armies might have similar units with highly beneficial auras that you want to keep in the game, if not the fight, or that are squatting on a key objective. Anytime a unit is where it needs to be and does its job outside the movement, shooting, and assault phases, it is a candidate for going to ground repeatedly.
4. When all the shooting has to go at one unit.
Smart opponents will give themselves options for what to shoot so they aren't left engaging useless targets or over-dedicating firepower. Sometimes, though, you just don't have the range, line of sight, or whatever to hit the right targets. Sometimes one poor squad of Marines will be stuck in front of their entire army's guns, or even just the guns from a whole bunch of guys on the other side. If you are pretty sure they are going to have to pour enough firepower into you to kill you because they have nothing better to shoot at, going to ground early on may not be a good idea.
This isn't anything like an exhaustive explanation because, like many decisions, whether or not to go to ground is very context-sensitive. It depends a lot on your chances of living (or having enough members to be useful), the mission, what else is nearby to take over the role, what else your opponent can shoot with, etc. However, it's a good idea to stop and think sometimes- would another +1 to my cover save change things much? 50% more models surviving on average can be a big deal.
