Our minds need to use some definition of personal identity in order to function: open, empty, and closed individualism are the alternatives that you can try to make your brain follow, though we're pretty strongly hardwired to use closed individualism by default and that's very difficult to overcome.

The choice of personal identity doesn't necessarily alter our predictions, but it can (temporarily at least) change our values and thereby behavior: if you believe that you are everyone, then you are much less willing to hurt others. It may also affect things such as your happiness, if it makes you feel more connected with others or if it makes the risk of your own death feel like less of an issue.