Many armies in 40K have a special rule that players must master if they want to used the army to its full potential. Eldar live and die by Battle Focus (well, except for Wave Serpents and Wraithknights). An Ultramarines player must know the moment to employ each of their combat doctrines. In the last codex, Space Marine players had to learn to use Combat Tactics. In the current codex, I think the ability to use Combat Squads separates the best Space Marine players from the average ones.
Now, I'm not saying that I'm one of these best Space Marine players. My tournament standings put me at below average, but I am trying to improve. For most of my Space Marine history, I've kept my squads at full size. In my mind, this made them more durable and kept their firepower concentrated. I've been experimenting with splitting my squads lately, and I've discovered that I was dead wrong about both of these things.
The most obvious advantage is that you can divide your fire. In the case of a Devastator squad, this makes sense. Four heavy weapons will often be overkill against one target, so you can split the squad and engage two targets. You can always fire at the same target if necessary. This gets even better for Tactical squads, since their special and heavy weapon rarely want to fire at the same target. Moreso, the heavy usual wants to stand still while the special moves. If you combat squad, both weapons get what they want.
|The combat squad with the meltagun breaks left to target an enemy Rhino while the combat squad with the multi-melta takes up a static position in the ruins to the right.|
Another obvious advantage is that combat squadding gives you more scoring units. If you have a lot of objectives to hold, it's better to have two independent units than one large unit. This is why Combat Squads is most often touted as a useful rule.
There is a less obvious (it was to me, at least) advantage to splitting your units: damage control. There are a lot of units on the table now that specialize in overwhelming firepower. They search for large, important units that are worth concentrating their shooting on. Five Tactical Marines rarely make such a good target. I was really surprised how powerful this was once I tried it. Centurion Devastators are very popular around here, but their massive number of grav-cannon shots aren't too impressive when they're just blowing away five Marines a turn. Even my Thunderfires are depressed when they can only hit small units. Combat Squads puts a hard cap on how much damage your enemy's super units can do each turn.
|These Centurions have way too much firepower to aim at the 5 Marines in front of them.|
The same can be applied to enemy assault units. As a Blood Angel player, I know that assault units are looking for a juicy unit to beat up. Ideally, the target will be able to survive an assault phase so that the assault unit will be safe during the enemy shooting phase. The worst thing you can do to assault specialists is force them to assault a relatively unimportant unit that they will wipe out in one turn. Then they get shot and the process is repeated. This is extremely frustrating to the assault player, and it's really easy to do with careful placement of combat squads.
|The combat squad with a flamer disembarks into the rubble, preparing to receive an enemy assault, while the combat squad with a heavy bolter takes the high ground to the rear.|
All of the above examples apply to any MSU (many small units) build, but Space Marine Combat Squads give you an extra advantage. Combat squads that break off from the same squad can ride in the same transport. This is awesome! It allows your small units to maintain their mobility without having to buy a transport for every squad. It also gives you many options when you disembark. Even if your Rhino gets blown up, you can place your combat squads in positions to frustrate your enemy's plans. Often, I'll put the special weapon squad forward to absorb any incoming charges while the heavy weapon squad will move back and take up firing position.
All of the above are reasons that I am learning to love Combat Squads. I have found a downside to it though; it reduces the impact of Veteran Sergeants. Rather than giving LD9 to the whole squad, he only passes it to the 4 Marines that follow him. There's a simple way around this; don't upgrade to a Veteran. It's really not worth it if you plan to split your squads.