It depends on what game. Note that these are not just pointless money-grabbers but games I actually enjoy a lot and have played for years.

In some it's the feeling of becoming powerful. In some it's out of frustration knowing it would take days doing something I can spend a few bucks on to skip. In others it's about collecting stuff. And in some it's solely about supporting the creators.

I also use it as a kind of reward. I treat myself with a big legendary chest.

With that said, I'm 100% guilty of falling for the artificial sensation they've manufactured with all the animations, sound effects and deals they put in front of me. These companies are experts on making IAPs "feel good".

Finally, I have a lot of money (made a big exit) so it's not really a big deal for me.