What is the point of the transistor to drive the relay coil
The transistor and the resistor might be needed because the input can not supply enough current to drive the relay. D4 protects the transistor from a negative voltage at the input.

D6 protects the transistor from a negative power, and prevents spikes to enter the power rails.

A relay that is switched off produces a current pulse that must be dissipated somewhere, otherwise it will destroy the transistor. D1 could be intended to protects the transistor against this pulse, but IMO it is placed wrong: the diode should be in parallel with the relay coil.

I can't see any purpose for D3, unless it has something to to with components connected to the vertical wire that are not shown.

All in all this looks like a circuit produced by a committee with too much open agenda's and some closed ones too.