Reasoning behind open relationships?

I saw a tidbit on open relationships and originally didn't understand how it could work. I asked my boyfriend (of 2yrs) what he would do if I wanted that and he said he would be completely fine. He said he would think it was hot. I don't know what I should make of this. I've always wanted to try out girls but opening something like ... idk if I could handle it. Could someone explain the reasoning/thought/feelings behind open relationships? I'm very interested and curious about the concept but know I don't quite understand.