A few possible reasons, just off the top of my head:

1. Representative democracy is a pretty weak constraint on political action, all things considered.
2. Cognitive diversity is generally a good thing for decision making bodies.
3. Role models are important.
4. Sometimes symbolism matters.

I think 1-3 are fairly obvious, but can provide more argument/detail if you disagree. I'm probably least sympathetic to 4. YMMV.