If you have the same password on multiple sites, you need to read this post.
What Problem Are We Solving? Password reuse is a massive issue when it comes to our online security. We can't trust that websites are storing our passwords properly nowadays; even well-known companies occasionally mishandle passwords (example 1 , example 2 , wall of shame). If you use the same password for, say, your Neopets account and your bank account, what happens if your Neopets password gets leaked?
Email providers like Google, Microsoft, Yahoo, AOL, and the email services included with Internet Service Providers make it easy to get your own identity on the internet. Generally, this is a good thing; everyone needs an email address nowadays. We use email to identify ourselves to most websites and apps, and so the more connected we get, the more essential our email addresses become.
What's The Problem? The more ubiquitous email becomes though, the more power our providers have over us.