Someone asked me such a basic question the other day that I was almost stumped for an answer. I had mentioned that we have broadband access through Comcast cable, and she asked why I didn't have a comcast.net email address. I've had a Gmail account for so long I'd almost forgotten why I got one in the first place. The answer is that if I ever want to change ISPs I won't have to change my email address. If you use the email address that your ISP assigns to you, then it's a bit of a hassle to switch to another provider because you have to notify not just friends and family but all the websites you have accounts with.
In my case that meant notifying three banks, 4 credit card companies, the local library, my doctor, two newspapers, Walgreens, my health insurance company, Amazon, Barnes and Noble, my cell phone provider, and what seemed like a zillion more.
So my recommendation to everyone is to get an email account set up that can follow you around. Gmail is great, but there are others, like Yahoo, that will give you a free account.