Learning Vassal is not exactly easy (although this may be useful to newbies...); so why bother? It's not really representative of 40K because you don't learn important skills like judging distances, assessing the battlefield from a proper perspective, playing for speed, moving and positioning units correctly, etc. So what's the point? There are several, really.
1. Your schedule
Yeah, yours. And also that of the FLGS you go to for games. Now, maybe you're lucky and your store is open until 8 or 9 PM and you never have any problem getting a game in, but for many of us, scheduling our life around plastic spacemans is just not feasible with work, school, etc. Vassal doesn't care about any of that- any time you get on, there are probably some folks from somewhere in the world looking for a game, and if you're hanging around the 3++ Chatbawks you can probably even find some folks who aren't jerks.
2. Reviewing Games
This one is easily overlooked, but I find it invaluable. Unless you are compulsive-obsessively filming every game of 40K you play, your ability to go back and re-analyze your losses (or wins, even) is limited by your very fallible human memory. Where you remember things being- or how you remember them happening- may or may not be anything like the way they actually were. With Vassal, you don't have to worry about that- if you save a game log or take screenshots (both rather simple affairs with the program), you can easily go back and look at what you and your opponent each did in sequence, figuring out where you went wrong and what you should have done instead. For a competitive player this is incredibly useful, as you want to be learning from every single game if possible.
3. Different Opponents
Unless you live in a major metropolitan area, chances are you only have access to one or two game stores within a reasonable distance of your home, which means your selection of opponents may be fairly limited. There may be armies (or, at the very least, army builds) you've never even had a chance to face- and that's assuming you're lucky enough to have a base of players who are interested in playtesting and competitive play at all. Vassal gives you access to a pool of folks who, by the nature of the program, are going to be more concerned with good play and good armies than they are the hobby aspects- which, for a tournament player, is exactly what you're looking for.
4. Try Before You Buy
Let's face it, 40K is an expensive, expensive game. If you're considering building a really weird army list, or starting a completely new army, it doesn't hurt to give it a whirl and see what you think of the playstyle first; maybe you'll love it, maybe you'll hate it, maybe it will be completely terrible. Obviously you can do this by proxying in tabletop 40K, but that brings up the issue of "are those Orks supposed to be Terminators or Paladins? And are the Big Shootas Halberds or Falchions this time?" Vassal lets you bypass all of that, and only rarely will you have to counts-as something online unless you are playing an extremely new army that hasn't received model updates yet.
5. Play Super-Cool and Sexy Internet Celebrities!
Because you know you want to. :3
