How do you help your team with roaming, and who should take the roaming role? I've seen some mid AP carries roaming, but that's a risk because they are leaving their turret unguarded.
Having a roamer and a jungler allows you to effectively have three solo lanes, which gives your carries faster experience gain. But that's really just a side-effect of the roaming strategy. The roaming strategy is mostly defined by having a non-jungling champion with no "home" lane. That distinguishes it from simple ganking.
The role of a roamer is to enable the other lanes to get kills either through a combination of CC and the carry's damage, or by having enough burst/surprise damage to effect a kill. The basic idea is that, in combination with the jungler, there are so many ganks going down that the other team is going to fall behind in their lanes. Thanks to the experience advantage, merely damaging a lane or forcing them to blue-pill can generate an advantage.
Good examples of this kind of roaming can be in seen in the IEM Hannover finals with Moscow 5 running a roaming Alistar against Dignitas: Game 1 with the roaming Alistar and a Lee Sin jungle
The downside is that you need to roam with someone who fits the role, which is not a lot of champions. Low-risk roamers are those who don't need a lot of levels or gold (since you won't have much of either) to still be effective. Alistar and Taric are good examples of this, though Alistar is by far more common in the roaming role. High-risk roamers are champions who do need the gold (but not necessarily the experience) and are usually assassin-style character. Twitch is a good example. Talon can also work, but his lane power is so strong I can't recommend doing it.
Sometimes you'll need to roam even when you haven't specifically selected for it. This is usually because you've either destroyed your opponent's turret or because your lane is truly hopeless. However, before choosing to roam, make sure you read everything above... an ineffective roamer is just an exp leech and is rarely a threat in the other lanes.
Finally, there's the situation where you have a champion who can leave their lane unguarded. This is usually because they can either push the lane toward the opponent's turret, and thus have some time to roam, or because the opponent isn't capable of pushing very hard (or both). Nunu is fairly effective here because of the speed boost from blood boil: He can make it another lane quickly and has decent CC. However, it's rare that Nunu will spend the inordinate amount of time outside lane to be truly considered "roaming."
Usually you'd Roam after there's nothing left for you to do in the lane (ie, both first turrets are down and your lane is pushed up and you don't want to over-extend), but there are some champions that roam as their role, such as Twitch. The benefit of this is that it allows their laning partner to get solo xp, and the roamer can gank other lanes to help them get kills. Even without getting kills, the Roamer can scare other lanes so that they don't get too aggressive or so they have to buy wards.