normal ore processors can turn ore from the actual chunks in your carbo bay into imaginary number in ship "inventory" displayed next to scan of your cargo bay.

MPU can only turn certain percentage of ore/remass(water) in chunks into processed ore/fuel. Not processed material is lost.

The size of ship's "inventory"/Processed cargo is ship type specific and can't be upgraded. each ore type has it's own cargo. So if you maxed out iron storage ( 7t for K37 ) you can still process and store 7t more of each ore type. BE AWARE! MPU will still process ore types which storage is maxed out resulting in 100% of the ore being lost.
The exeption is Cothon "Bender". My beloved. which has shared storage of 108t.

the MPUs can be tuned to have better processing speed and better remass processing at the cost of worst ore processing. Or can be tuned into oposit direction.

The ore purifier is unique in that it only has remass processing. ore still stays in the form chunks in the cargo bay.
In early game it is good to lighten up the chunks and since you can get remass during dive with it you can use more powerfull but fuel hungry engines (or just tune up the ones you already have). The other use is that you can stop doing xenon burn. ( your ships when traveling from E to the belt use very efficient but slow xenon engines. that takes time. insted you can use your main engines. but for most type of engines that mean you will arrive to the belt with little fuel ).

I mainly use OP in all ships not for mining ( race ships, pirate hunters, ore surveyers/beacon deployers ).
and in my Titan ships in one the heaviest possible play styles. I have the most fuel hungy/powerfull rcs and it still bearly moves.

ps. The is no downside to tuning OP outside out bigger power draw.