It is too easy to fake e-mails. You can set up a catch-all e-mail domain and spam the registration like that. I am not a fan of giving my e-mail nor collecting other people’s e-mails.

My current message contains the following:

Please leave a short message (a sentence or two is enough) stating why you would like to join this instance and I will accept your application as soon as possible. The purpose of this form is to filter out spam bots, not to judge your motivation for joining.

It is not about them writing an essay to be let in. It is a very effective strategy to weed out spam accounts being registered in masse. One step is to make sure that the user made a cohesive sentence that addressees the question, and the other step is to check whether there is a sudden spike of similar new applications. Even ignoring the actual text, it is useful to be able to monitor whether you getting rate-limited bursts of account creations, and having the ability to approve/deny allows you to respond with less effort than if they succeed at creating the accounts.