Throughout the booking process, they'll reserve the seats for you (so you don't get sniped while entering your details). This changes the number of available seats on the flight, which can push it over a threshold into the next pricing bracket.
EDIT: bonus round: screw with airline algorithmic pricing algorithms so hard that all their models get so screwed up that they end up ditching the idea. Victory for everyone except the airlines. Use the arbitrage profits to push this agenda. I'm sure you'd get sued out of existence long before it ever worked (unless e.g. It's open-source and ran by individuals), but it'd be hilarious. This is the kind of stuff I'd fund if I were rich.
If you hold without ticketing you have a limited time period you can hold, furthermore if you hold and cancel above a certain threshold the airlines will get upset and effectively block you from their inventory.