Griz, paper patching improves performance in one important way - for hunting, you can use very soft bullets (even pure lead) to get good performance on game, without getting any leading in your barrel.
One of the problems with cast bullets is that to shoot them at higher velocities, they should be fairly hard - and this results in poor performance on game. By fairly hard, I mean 1:20 or 1:30 tin to lead.
For most hunting, you would want your alloy to be 1:40 or even softer (a big flat nose helps, too). But if you use bullets this soft, you may run into leading problems. There are several ways to get around this - the "cast softnose" is one, where you first pour a little pure lead into your mould, and then immediately pour in some harder alloy. This gives you a hard bearing-surface, but a soft nose for expansion.
Another, very traditional, way, is to paper-patch. The paper patch acts just like a metal jacket (in fact, the early metal jackets were referred-to as "metal-patched").
There may be some disagreement as to exactly how hard you want your bullets to be for hunting, and it depends a lot on what game you're hunting and how fast you shoot your bullets - but the point is, if you want to shoot soft bullets, then paper patching will allow you to do it.
If you're really interested in this, then definitely get Paul Matthews' book "The Paper Jacket".
