I'm officially not giving anyone poly advice since my own life is on the rocks, and you have had plenty of really good advice already. However, as to why do poly if you can be a happy mono?
1) My gf and I share many interests and passions, but not all. Both have individual interests that the other can sympathize with but not really share. These can be so important however, that sharing them with someone feels really exhilarating. If feelings of exhilaration turn into something else and there is a negotiation system in place for such occurrances, it feels a lot of safer to venture outside of the primary relationship to meet new people and make friends.
2) We have very different sexual needs on a very basic level. I'm primarily attracted to women, she craves male energies. BDSM is very important to her core identity, I'm sympathetic but don't really feel the itch. While I am mostly responsive to other's sexual ardour, she is very sexual and craves sex and sexual variety more often than I can reasonably provide.
3) We both have a disabling long-term illness, but she has it to a much greater degree, and the diagnosis is much more recent. She needs considerable time and energy just to manage basic life functions. I work two jobs, go to school, volunteer on multiple levels and have several hobbies. It does us both good not to have to fit ourselves into the other's rhythm, but to be able to seek companionship outside and alongside our life together.
Me: bi female in my twenties