What is the reason for all the gritty stuff on the surface?

Get roofing, in Macclesfield

maybe to help reduce the tar flowing in hot weather?

Stops the roll sticking to itself?

From Wikipedia

Roofing felt, one common type of tar paper, is a glass fibre or polyester fleece impregnated with bituminous material (tar, asphaltic bitumen); it is produced in roll form. In some cases mineral material (e.g. sand) is applied on one side to help prevent the material from sticking together while in roll form and to provide protection from atmospheric conditions.

Well done. You both win a prize.

…and to stop birds and roofers falling off.

You don’t win a prize for that.

I was roofing on the shed errr… roof and slipped. My slip has removed some of the gritty stuff. I was wondering if that would result in me having to do it all again next year.

The grit stuff is also there to give it a bigger surface area so it lasts longer in the sunlight- ie helps the heat dissapate, apart from shonky fitting this is one of the biggest causes of flat roof failure.
Used to deal with lots of claims for them in my old job- max you’d usually get out a flat roof is 8-10 years tops.
There were some fancy plastic bonded ones coming in back then but changed jobs so havent kept up.
Keep an eye on the patch that you’ve taken the gravel off.

It wont do it much harm, if its specific shed felt it wont last more than 3-5 years.