I think people often confuse Poker Tracker with odds
calculation software and the like. Yes, Poker HUD will show you stats on your opponents, but the same could be accomplished by keeping detailed notes on those you face. (although would be exceptionally time-consuming)
But, for the most part, PT is used far more for:
a) Finding holes in your game (ie, hands you play too strong or too weak)
b) Tracking your wins/losses (ie, which limits are most profitable/costly for you)
c) Providing neat charts to encourage (or discourage) you as you see your progress over time
Because all odds even out over time, a large database is the only real way to analyze your game. Going over how you played in an individual session can be useful, but might steer you in the wrong direction if you have an unusual run of cards.
Personally, before I started using PT, I was tracking everything manually in a database. That was a nightmare, and lead to inaccuracies and distractions during games. Now, I can concentrate on the real task at hand: playing poker. I certainly wouldn't be lost without PT, as I'm usually playing against people I have no data on anyway. And if you get too caught up in the numbers, you miss out on obvious information. PT doesn't tell you when someone goes on tilt, but a good eye at the table will.