There's a couple of reasons you might want to, say you have a fast connection but it's capped and expensive, you might want to use this for regular Internet traffic, you might want to direct traffic to sites like YouTube through a slower cheaper backup link.
Another reason might be that you want corporate traffic and Internet traffic to break out at different places etc, maybe you have a few lines...
Thing is, I see plenty of reasons to want to do this kind of thing, but getting into a reason to use this sort of thing is fairly specialist!