Some guys think men and women can't be friends, but that's nonsense. Female friends offer the obvious value of introducing you to her girl friends or being great wingmen, especially if your friends are attractive. Since there is no better way to seem attractive to women than being seen with an attractive woman.
But now I am seeing the incredible value of the role they play in emotional connection. As men, we don't allow ourselves to be as vulnerable around each other. Even if we want to, it's just not how most guys communicate.
For example, after my last break up from a serious relationship, my guy friends were there to shoot the shit, drink beer, go out with me to hit on chicks, etc. But conversations about how I was feeling were reduced to, "that sucks bro, but there's more chicks out there."
But with one of my best friends who's a woman, I talked more openly about how heart broken and sad I was, and she listened without judgement, without worrying about whether or not I'll lose my man card. Both types of friends were there for me in different ways.
Both dynamics are valuable and why we need different kinds of people to play different roles. I would even say that female friendships are even more important when you're in a relationship, because women lose attraction for their men when they are too vulnerable, but you don't have to worry about your female friend not seeing you as attractive since you're already friend zoned anyway. The only problem is that your partner would not like that you're being more emotionally vulnerable with another woman.