I know guys who have hunted for as long as you and dont know what grain or style their ammo is, or what model their 30-06 is. Age doesn't always go hand in hand with knowledge. But that's ok, we're never too old to learn
Velocity is important. It gets the bullet out of the muzzle and towards the target. If there is enough velocity, it makes bullets 'mushroom' when they hit stuff. If there is too much velocity, sometimes it makes them 'mushroom' too much, which with some fragile bullets will make them fail to penetrate very much.
How much velocity is needed depends on what the shooters needs are. In terms of trajectory, 2200 fps will get you to 200 yards no problem with little need to compensate for bullet drop. 2600 fps to 250 yards with no more than 3" of drop. 3000 fps will get almost 300 yards. 3400 fps gets 330 yards before it'll drop 3" from line of sight. Even with the fast ones, you need to know the range and where your bullet will hit at that range, in order to make a good hit. Flat trajectory is nice, but in todays age of laser rangefinders and ballistic reticles and/or elevation turrets, it's not very important anymore.
If the bullet has sufficient velocity on impact to expand, and has the construction to hold together to penetrate the vitals, then it doesn't really matter if it lands with 2400 fps or 2700 fps.
This really isn't that complex. A 257 Weatherby Magnum has about a 300 fps edge over a 260 Remington, Similar to what a 300 Winchester Magnum has over a 308 Winchester. All of these cartridges kill game, some of them do it more efficiently than others, some of them erode the throats on the barrel, some of them are chambered in longer, heavier rifles, some in shorter, lighter rifles. They all work.
It all boils down to putting a good bullet in the vitals from a sufficient caliber. Like I said before, I'll put bets on a good shooter with a 308 (or 260) outshooting a poor shot with a 300 Ultra Mag (or a 257 Wby Mag) when it comes down to hitting a target at long range.
here's a video you can watch. Even though it is not that long of a shot, 340 yards, it goes to show that you can make consistent hits with a bullet exiting the muzzle at 2750 fps. If I was shooting a 257 Wby, I'd have to do everything the same except make four less clicks of elevation
I hope this clarifies your question as to how velocity can be important in shooting guns.
http://www.youtube.com/watch?v=Y-Bh-ZuIN50