I do all kinds of things in free practice sessions in Career and often re-start the practice session because there is not enough time to work out everything in one hour. If i haven't sorted out a setup prior that is the number one thing i work on (usually with about 75% fuel load on Primes).
Next i usually sort out the time difference between the tyres, sometimes it's only half a second or so but sometimes it's a lot more.
Then i check out the tyre life by doing long runs (note this only works if you do 100% distance races because the tyre wear is not scaled down in free practice).
If you do full quali sessions you have to remember to conserve your tyres for the race, you only get 3 sets of each tyre for quali and the race, so restarting the free practice sessions helps with that as well.