What do you get out of making new friends and maintaining friendships?
People to talk to. I'm not what you'd consider to be a highly social person, quite the opposite actually. I'm not anti-social, I just don't always want to partake in social interactions. Gaining a friend does offer opportunities however to be social and such, and usually they've got similar interests to my own, so naturally it's easier to talk to them about things and get to know them if you share something you enjoy.
What do you get out of being in a romantic relationship?
Love. Both given and received. Intimacy is very often involved, but need not always be.
What necessarily separates your romantic partner(s) from your friends?
With a friend, you can share many things but there's often no feelings of love to be shared, or as it is often in my case, the love is simply one-sided because they have another love interest or just don't believe themselves to be compatible with you. There's simply little that can be done in that case. I don't search for the mythical exit to the friend zone when I get put there, I move on.
With a romantic partner, you can share many things and so much more. There's no longer any barrier, and you know that they love you as well. Intimacy is often a hallmark of romantic partnership, but may not always be. Some friendships are strong enough to allow intimacy, but this is not something most people would logically expect to happen, as it is somewhat uncommon and viewed in a bad light in a number of cultures. Generally speaking, most friendships that reach this level are probably romantic anyways...or implies that they have romantic intent, even if one does not wish to appear to do so. Romantic partners are often capable of sharing more with each other anyways, especially emotionally where it oftentimes really counts. This sometimes makes for a rocky road, but other times it makes life all the more livable and appreciable.
In essence, a romantic partner is ideally someone you consider to be the most treasured friend you have. Everyone is different though, and reasons for treasuring a friend are as varied as the colors we see, so it may not always be obvious even if it commonly is.