If you hand out the same email everywhere, eventually data brokers begin to connect the dots. If this email shows up somewhere, it means that they like this stuff, buy that stuff, they've been here before etc. All of that and more will be revealed when you create a new account with that email.
I consider that a privacy concern. Companies use data brokers to exchange data and build a more comprehensive profile of you, and your email address is a part of it. There's a lot more though, so escaping that kind of profiling is tricky. Using unique emails is a good start though.