What is the purpose of bonus points in Gin Rummy?
The most common way of betting money on Gin Rummy (according to this site) is to choose a rate per point (say $1) that the loser pays the winner - e.g. if you lose 70 to 220, you pay $150. When you're playing for money, there's probably an approximate stake you want to play for - smaller stakes will feel trivial (who wants to win one penny?) but larger stakes could be devastating if you lose. Without bonus points, a win could be anywhere from 1 point to ~150, and there may be no per-point rate which is non-trivial when multiplied by 1 yet non-devastating multiplied by 150. Bonus points - say 100 for winning - are a simple way to decrease this variance, since now spreads will be between 101 and ~250, so the worst possible loss is only ~2.5 times as bad (or good, for the other guy) as the closest possible spread.

As far as I can tell, best-of-5 matches tend to be played in settings where bonus points are not relevant, e.g. tournaments where all that matters is who wins the match.