I'll answer your question first. Why become poly? Well for many people it's just the way they are. They 'become' poly when they learn how to handle the multiple relationships with honesty and communication. Though there are those that enjoy more relationships because of a kink factor, one partner is into a kink that the other is not, typically adding another partner to make up for a lack in your (main)relationship is not the idea. Sure you may have different interests that not all partners share but, for example, I don't date BF because he likes to write out fan fiction with me that hubby won't.
Ciel is right in the fact that everyone has their own way of being poly, it's about deciding on what works for you. That changes and gets renegotiated as time goes on and other people join your circle.
As for the rest, if you feel your marriage is missing something, then that's something to discuss with your husband. Personally I don't think just adding more people will help that. It didn't help us, it actually made things more complicated and difficult. We, my husband and I, had to work on our relationship, strengthen it, work on communication, discovering what we want and need from each other and being able to ask for it. Now that we are doing that, poly is easier for us both. Hubby is mono so me being poly when our relationship wasn't good, was harder.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year