It's so you don't get a villager that recently moved out because it can take up the 10th spot in someone's town that they could have been saving for a dreamie
It's been mythbusted as being a surefire way to actually clear a void, but it cuts down the risk of having a voided villager ended up in someone's town. Common courtesy now, really.
When a villager hasn't been adopted but still move out, they're in your void. That means that they can move into the towns of people you StreetPass or wifi if they have a free spot.
One of the ways that was recommended everywhere some time ago was visiting someone with a full town. The idea was that the voided villager would try to move into that full town, but they'd be deleted due to not having enough room to move in.
I think that's been shown to be false, though, so right now the only way to clear your void is by visiting someone who doesn't mind getting a villager and has at least one free spot. If the villager moves into that town, they'll no longer be in your void.
It's pretty simple, but a lot of people dislike getting random move-ins (especially when they have 9 villagers and are saving that last spot for a dreamie) so a lot of people prefer interacting only with people with a clear void.
By the way, if you're a cycler and someone visits your town to adopt a villager, they won't get a voided villager. They will always (I think) get the one they've adopted instead.