Without trying to dig out my paperwork, I think that most commercial mortgages require an assignment of leases. It isn't so much collateral as it is a "prepaid funeral".
If my business dies, the paperwork is in place to allow the bank to start collecting the rent on the property while it tries to sell it to another investor. Otherwise, the tenant has an agreement solely with the investor.
Having a lease assignment in place gives them the written proof to say "you owe your rent to me from now on" when things turn sour.