A Merchant that accepts MCA Funding has an Attorney? Not in my experience.

The main purpose of a Check 21 is that is does not require any additional action by a Merchant.

IF you are charging a PSF that has already been approved by the merchant when he/she has signed the funding documents.

Yes, they could a wire or an ACH which takes 1 to 3 days - Check 21 is just another option.
Many business owners have never initiated a wire or an ACH - additional a business owner may have "Bill Pay"
but not ACH which with many banks is an additional service that they have to apply and obtain approval.