pre season

Question
What is the purpose of the pre-season games? Does the outcome of the pre-season games determine something for the season or are they just for practice?

Answer
Preseason games have no bearing on the official NFL season.  Pre-season games do not impact standing, nor do they affect statistical records.  The purpose of the pre-season is:

1) Practice (as you indicated).

2) Evaluation. The coaching staff of a team gets to look at new/younger players, and new player combinations (such as shuffling the offensive line assignemnts) without risking regular season standings.

3) Revenue.  Pre-season games produce significant reveues in the form of full price ticket sales, concession sales at stadiums, and broadcast revenues.