There may have just been a sharp edge created by a tyre lever on removal that needed the grinder. They should have mentioned this to you though.
bloody hello a angle grinder
You might find its to clean the lip of the wheel so the tyre makes a good seal. Normally they use a brush atta hment which cleans it up nicely else it would leak on the rim.
I had dreadful visions of the 'Chainsaw massacre'!