On top of what Raga mentioned (keeping a place in your heart for someone without needing to put your life on hold) I think a big thing is the constant support. It's like being part of a big family where everyone cares for everyone and helps everyone through things. When you're used to being all alone even wen surrounded by people, it means a lot to have partners or metamours you can trust, feel close to or just tell anything to.
Also, in a monogamous relationship, you can't ever share the giddiness of falling in love again, or be comforted when turned down. Not by a partner, at least, only by a friend, and as good as friends are, partners allow you to remember that there are people who did love you the way you were, and still do, and therefore you're not defective or flawed when you get rejected. It's so easy to feel bad about yourself when you are rejected, but existing partners allow you to feel more secure about yourself and take things better.
Me: 31F, straight
Seamus: My husband, 32M, straight
Dragon: My boyfriend, 29M, pansexual, married to Fox
Fox: My boyfriend, 29M, homoflexible, married to Dragon