Google apps not validating mx records
All it does is pre-populate the MX records for you, and I think adds some default CNAME records as well to help beginners out.
Alternatively, if you are not using the existing nameservers for anything (perhaps these are just your old domain provider's servers, if you transferred to Hover recently), you can use the Edit option to update these to Hover.
You can verify a couple of different ways: using a TXT record or using a CNAME record.
You also need to add Google's MX records, and edit the "mail" CNAME record.
But I think the overwhelming argument for the GApps records is because so many people here use them.
Time and time again, I hear some of the "pros" here (or at least people who seem to know a heck of a lot about these systems) say that if possible for the situation, go with someone else hosting the mail, specifically Google Apps, because it reduces so many headaches.