It has its uses,but you don't need it if planting in gravel..I used to mix it in when potting on seedlings to aerate the compost..you don't need to wet first..I'm not certain if it made much difference,as they were fine when I ran out of it,so I didn't buy anymore..It won't do any harm,anyway..

as bloomer says it adds bulk to the compost helping drainege etc. I use it occasionally if the plant needs sharper drainage than most. or I use sand/gravel.

Bulba and I never use just grit, which is cheaper and gives the same result.

Normally mix it dry 10-20% with potting compost. It does aerate the soil and improve drainage but also lightens the compost, whereas grit makes it heavy. It has another benefit - it is extremely porous and can help retain moisture releasing it slowly back to the roots.
Vermiculite (coarse size) does a similar job but has the added bonus of holding onto nutrients too.
You don't need to use it but I guarantee that many plants (okay, I mostly mean Buddlejas) will benefit from your using it.

Ah alpine plants mostly like a lean and mean mix... totally unlike Buddlejas. Guess it really depends on what you are growing in the pot.