What the clause does
Forfeiture is the landlord's remedy for a breach: the lease comes to an end and the flat goes back. A mortgage secured on that lease disappears with it, which is why lenders care.
A mortgagee protection clause obliges the landlord to notify the lender before forfeiting, so the lender can pay the arrears, remedy the breach or apply for relief. It converts a catastrophic risk into a manageable one, and lenders now treat it as a requirement rather than a preference.