ISO is just a copy of a disc.
It has no purpose. It's like a copy of a file you have on your computer, the only purpose is to HAVE a copy, in case you need it (backup for corrupted data, lost the first copy, etc.)
What you do with them is your own decision.
PS3 can play backuped games (not ISO, but extracted data from disc) from HDD, but there are very specific requirement :
- You need an OLD PS3.
- You need to install a custom Firmware
- Your current Official Firmware must NOT be over 3.55
- You can't downgrade to 3.55 if you have a new PS3.
You seem to be already on CustomFirmware 3.55, so you can dump/extract the content of your games to HDD and play them from there, by using a Game Manage (most popular is multiMAN).