lets make thigs quite simple if i can.

ir's are kind of a "snapshot" of a rig,in this case lets say a cab and a certain mic in a certain position.

those snapshot can be used like "cab simulators" in various ways.

epsi is one of those ways (its a machine used to "read ir's")

depending on the quality of the studio wich make those "snapshot" results may vary from a decent cab sim to a super
accurate reproduction of a cab and a mic.

you can use irs in place of others cab sims like the ones on 11r for example.

lets say that ir's are the future of cab sim (even if its quite some time theyre out there).