Besides allowing people to play more hands because it goes quicker (thereby increasing the hourly profit of both the
poker site and the winning player) and compensating for the difficulty of reading chip stacks in a 2D environment...
Keep in mind that displaying the pot amount encourages people *not* to pay attention. They can be in la-la land and still see exactly who did what. This allows people to multi-table effectively (because they don't have to watch the action) and it also allows people to watch television or tend the kids or do some other distracting activity.
Only, if you're used to playing like this, and you move to a live game,you'll have a hard time adapting to the fact that you need to do it allyourself - since you won't have trained your memory and computationalskills. Asking the dealer what the size of the pot is all the timeisn't very practical, IMO.
Well, playing with a HUD lets you play a lot more tables so you see a lot more hands and pick up the math a lot quicker. With online tables seeing hands at twice hourly rate as live tables, and people like Zachvac playing 20+ tables, an online player can often play as many hands in one year as a live player does in his career.
When an online player moves to tables, the math and situations don't change, so he has a good foundation to play. He might not see that someone is in 44% of the pots and c-bets 73% of the time, but he will notice that the guy is "in a lot of pots" and "c-bets all the frigg'n time" and he'll adjust his play accordingly.
Really, I don't think observing and getting a feel for players is apt to be an online player's weak spot. I think boredom (from the slow pace of the game) and giving off
tells are going to be his initial problems.