Are you saying that having 300K Caps in your inventory was useful? I am pretty sure it didn't do anything for you. Who needed to buy anything outside of a few schematics and maybe some rare ammo here and there.
It was definitely useful and it definitely did a lot for me. Having ridiculous amounts of money is
always useful. It cost me over 5000 caps to repair my Vengeance Gatling Laser each time, which I used
regularly. If you've used that thing then you'll know yourself that it's durability is.. lacking. Same goes for the Eugene minigun, which I also used often. Power armour is also quite expensive to repair, although thankfully it's far more durable than the previously mentioned weapons. So that more or less covers the repairs.
I also had to spend a fair amount on Electron Charge packs, I'm sure you can guess why. Although I was alright for 5mm ammo, since technically I never carried 300,000 caps in my inventory. I carried it in 5mm ammo, which I could sell for 0.9 caps each
(incidentally exactly the same as 5.56 ammo, big hint to what I was doing there lol) whenever necessary thanks to my high barter skill.
The caps were useful in that it would have have been impossible to play the game the way that I did without them. Barter is useful. Perhaps too useful.