The unique email constraint determines whether constituents can register with an email address that is already in the system. By not forcing the unique email constraint, you are more prone to having duplicates in your database which may cause other issues down the road if they aren't resolved. Most clients enforce the unique email constraint but your needs may be different.

For more context, here is an example scenario:

6 weeks ago, a constituent visits your site and decides to make a donation. They complete the donation with an email address (test@email.com) and a new record is created in the database and the transaction is attached to the record.

A couple weeks later, the same constituent ends up on your site again. They fill out a survey with registration information like their home address as well as other personal information about themselves. They supply the same email (test@email.com) and submit the survey. A new separate record is created in the database and now the constituent has two unique records each with their own constituent IDs yet with the same email address.

If the unique email constraint was enforced at the time that the survey was submitted, the system would recognize the email address as already existing in the database and would attach the survey interaction to the existing record.