I used TA for ground school. I don't think it covered flight time, but I know that at my base (Griffiss AFB) I basically got ground school for free via tuition assistance. I know, however, that one of the incentives for being at the Griff when I was there was that the AF would cover 75% of education costs, and the base would cover the other 25% of the costs. Just about everyone got their Bachelors or Masters degree because of that deal, but I used mine on ground school instead.
They also told me that since "It's a flying Air Force, we encourage everyone to have as complete understanding of flying as possible" and that was also why they covered so much of ground school.
But that was 14 years ago, so I suspect the rules might have changed.