Back to the basics: What's so great about a framework?

Question

I'm brand new here and I'd like to start off by asking about WP frameworks. I've seen quite a bit out there, so I'm confused about what to choose for my site.

So here are a few questions:

1. What is the advantage of a framework in the first place? Specifically, what are the advantages of installing frameworks such as Woo, Thesis, Genesis over designing something for a Wordpress base install directly?

2. Are there are any frameworks that are not recommended? I've seen a lot of Genesis bashing in other forums, but I don't understand why this is the case. So I'd like to know what you use and why.

Thanks, Ray