"My husband and I recently did our wills through an online will kit. Do you think it's a necessary step for the witnesses to sign an Affidavit of a Witness to a Will?"
Yes, I do think it's a necessary step. Why would you choose to skip a step that the kit tells you to complete?
Assuming that you have made your will correctly and have had it witnessed correctly (two very big leaps of faith for online will kits, but that's a story for another day), then your will is valid without the Affidavit of Witness being attached. However, your will can't be probated without the Affidavit, so leaving off the Affidavit is a mistake.
When a will is sent to the court for probate, the court needs proof that the will was properly signed and witnessed. By "properly" I'm referring not just to the fact that each page was initialled and the last one was signed, but also to issues such as the person signing being of the age of majority and being of sound mind. This proof is provided by the Affidavit of Witness.
If you don't have it signed now, once you pass away your executor will have to find one of the witnesses and have it signed. This won't necessarily be easy, as witnesses may move away or lose touch with you. Or they may pass away. Why cause this kind of problem when you can prevent it by having the Affidavit signed now?
It makes me nervous that you are questioning parts of the instructions provided to you for setting up a proper will. You may be skipping over information or steps that are vitally important to a valid document. Please do not make the mistake of assuming that you don't need the Affidavit because your will won't need to be probated. You can't know that ahead of time.
My experience has been that some people mis-use will kits. They don't want to bother getting advice or paying a lawyer so they wilfully ignore potential issues and problems. Those are the people whose families pay for it later. You can produce a valid will using a will kit, but only if you have followed all of the instructions provided by the kit, including the advice about getting the Affidavit signed.