purpose of dual rca inputs on mono amp?

Could someone please explain why a mono bloc amplifier would have dual rca inputs.

They are probably connected directly together for biamping. One would be connected to the output of the preamp like normal. Then, instead having to run another interconnect from preamp to the other power amp, you just use a short interconnect between the two power amps.
Maybe one set is actually for "output" to another amp (for bi-amping). That's my guess. Contact the manufacturer to be sure.
Many mono-blocks make use of standard stereo chassis. As such, they are produced with two input jacks for stereo operation. Since most manufacturers and customers would question a product with an empty whole in the chassis, they simply fill it with the appropriate jack and tie it into the circuit. Not only does it look better, they can claim "added versatility" as mentioned in the posts above. Sean
Another approach that some have taken (such as in the Marantz 8) is to have one input bandwidth-limited for listening and the other wide-open for testing purposes. Of course, one could use either (or both) whenever one wanted.
one might be phase inverted.....just a SWAG.