Why diode use in parallel?

1.

Redundancy: If one diode breaks (in a way that makes it an insulator), the other diode(s) will do the job. In case of that kind of diode breakage the circuit is like an OR-gate: If diode1 OR diode2 OR ... OR diodeN works properly, then the circuit works properly. If it is important to make sure that the diode does not wrongly conduct, then the diodes should be in series.

2.

Reducing the forward voltage: In power electronics high currents need to be conducted with parts that are as ideal as possible (concordantly in this case each diode should be able to carry at least the full current, that has to be transferred through the group of diodes, in order to avoid a cascaded cataclysmic thermal runaway due to a decrease of the forward voltage of the overheated diode).