When you forget a password, what do you do? You click "forgot password", they send a link to your email, and you enter a new password, right? [1] Which means that if someone can get into your email they can get into anything. Sites get hacked and lose passwords all the time, so if you share passwords between sites it's really worth it to have a separate one just for your email.

[1] For me about half the time when I see the "your password must have one capital letter, two numbers, ..." I remember the password I used. I wish people would put the password creation restrictions on signin pages. Invisible at first, but showing up as a help message once you enter your password wrong once.