[noxer]

There is no such thing as a "write only DB" Who ever has control over that DB physically can tamper it. From external such a DB would be a black box. You store something and read again to check if it has been stored but whoever is in controller can just drop it after wards anyway. You have no clue whats actually happening. The black box could contain 2 DBs you query one and everyone else queries another.

A public DTL has a public state unlike a normal DB everyone knows the complete DB and its distributed. If you write to it you can read form anyone else including your own node to verify it has in fact been written. Instead of trusting who is in control of the read only DB you trust that a no majority of nodes collude to tamper. Its important to know that there is a negative incentive to collude because almost all participant are participant in such a system because they theme self want to use an immutable ledger.

This does not apply to BTC or similar blockchains where the people (miners) in "control" (collectively) are not the primary users of the immutable ledger. Because there is a false incentive (money/block reward) to participate even if you have no interest in using the system at all.