Hey

The basic reason to provide preparatory course in IIT is to benefit SC/ST/ PWD candidates with seats which were already reserved for them but it can not be assigned to them directly as hey have not cleared the JEE Advanced cut off.

Now these seats can not to be assigned to general category as well because IITs have to follow the reservation followed by the Indian constitution.

So to get the quota candidates admitted in the IITs and to make them good enough to be there preparatory courses are run to polish basic concepts of physics, maths and chemistry.

Hope it helps.