Why would I (landlord) receive a letter of discharge of debtor (against my tenant)

Because you were probably listed, as required, as a creditor or party in interest.  The lease is an asset of the bankruptcy estate and also an obligation of the debtor.    If it is a month-to-month lease, this is really insigniifcant.  But if, for example, the debtor had several months left on his lease when the bankruptcy case was filed, he would be discharged from the obligation to pay on the remaining lease term (but of course you would have a right to seek to evict him/her if he/she failed to pay their rent).