So, in advice columns all over the net, including this one, there are reasons galore for why you (as the GM), or the players themselves, should not split the party. these reasons are numerous, well thought out, and very good reasons to not split the party. However, there are times when it is good to split the party. Like all rules, the rule of keeping the party together has its exceptions, and as the GM it is your job to throw them in there and spice things up a little bit. So, below, I'm going to go over a few reasons as to why you would want to split the party up. You may also want to read this for some advice on how to handle it once the party is split up.
My Time To Shine
Alright, lets face it, at every game the focus of attention will primarily follow 1-2 of your players. These players may not be being dicks about it, they're just being more active. They take the lead, and shortly after the other players begin to fall into subordinate roles. This is basically just human nature, when led, most people follow. Sometimes they'll buck the trend, but in most RPGs this trend is further restrained by the fact most gamers don't want to be dicks. Not to mention, gamers can be a bit less social then others and just following along is easier. After all, who wants to be responsible for the fun of 6 people?
However, by splitting the party up you can give the spot light to those other players. Give them their time to shine, and a boot into center stage. Some of them will recoil and flee back into the darkness, and you shouldn't force them back out, but others will really shine once they are given a chance to really show through. It can also show just what that player brings to the party, as you custom tailor challenges to that one character to really show where it is a bad ass. Is the quiet player a Devastator marine who excels at wiping out hordes of enemies? Put him against a mob of weak enemies and let him shine. Are they a one on one specialist? Give them a big one on one duel. The point is, this is the best chance to give them the full stage, and everyone's attention, make them want it, make them love it. It will draw them out of their shell in other parts of the game too.
Establishing a Personal Villain
Alright, lets face it. Taking on 4-6 PCs at a time, and establishing a villain, is damn hard work. This is even harder when it is only supposed to be a villain for one PC. Beating the PCs is often, well...bad. However, when you split the PCs up it is much easier to establish that one on one villain. Put the targetted PC in charge of a group, and then slowly kill them off one by one while the PC is "powerless" to help it by the villain. Beat that one PC in a one on one fight. Make it a time for the PC to shine, let them make an impression, but also have them lose. Nothing gets PCs as fired up about getting revenge as losing, especially if it is in a painful way like watching a lone mech pilot rip your team to pieces before leaving you free-floating in space because you're not even worth finishing off.
Cover More Ground
This is the one that most PC groups rail against. Give them multiple objectives on opposite ends of the map, and they'll do one and then the other. Put a time limit on the objectives, and they can call foul because you're forcing them to choose one to fail. It amazes me how genre savvy is so set in most players minds with the idea that splitting up is the road to ruin, that it doesn't even occur to them. You may have to point it out to some people that they can do it. Just be sure to make sure the objectives are ones that can be handled by less than the full party. Start off by doing some easy ones, and then sprinkle in places where they need to make a choice.
The real benefit of this is that it can really up the tension. The PCs try for the sweep, and in doing so they increase the chance of total failure. Every roll counts for more, because they don't have the backup that they normally have. When they pull it off, reward them. If they fail, make it spectacular so that the story is still memorable in a good way.
There are more reasons to split the party then what I listed, those are just the first three that came to me. The ideal way to split the team is to make it a choice, however doing it on a mandatory basis is ok at times. You can also do it for some big scenes to give player control over all facets of a big battle, and to show the other players that big battle.
When splitting the party, it can also be good to give the other PCs some NPCs that they can control. It keeps them involved, while still making it clear who is the 'main character' for this scene. You can also get much more spectacular death scenes from a Player controlled NPC than one you are playing. It always seems to carry extra punch when you remove a player from the field, even if they are only controlling an NPC.
So, did I miss anything/ Thoughts or experiences with this you'd like to share?