What are they for? Never seen a 10mm fork are they used on normal qr drop outs?

Rear wheel standard quick release swop out. 9mm is for front.

I may be wrong but I think you can use 10mm skewer on the front of normal QR… like the DT Swiss one maybe?

Do they make much of a difference?

The front is 9mm, you’d have to widen normal dropouts to fit a 10mm.
The rear is 10mm, the are just thicker Qr’s that do away with the need for hubs to provide the means of resting against the dropout.
Many of us couldn’t tell the difference between these and bolt up’s!

Superstar do a decent set – Here

Don’t forget you will need to change the endcaps/axles on your hubs…

Jay

Why not just use a 9mm through axle on the front?

I’ve one of these 10mm axles running in qr forks.

I’ve got Superstar 9mm front and 10mm rear QR axle. They work well but I can’t say I’ve noticed a big increase in stiffness.