Why is the hard drive of a computer sealed in an airtight case?

Answer 1

Hard drives use what is called as the disk platter and a head for storing and reading information respectively. The platter are circular disks which hold data and the head read the data off these platters. The head is floating very close to the platters reading and writing data (using magnetism). sometimes,.the head may touch the platter, taking off the aluminum surface coat which actually stored the data, causing data corruption. Also the head needs to freely float without any jerks or air circulation movements. Thus, the hard drive is sealed internally in a air tight compartment. It also protects the platters from dusk, moisture and smoke.

It must be noted that not all hard disks work like this.

Answer 2

This is to keep out dust.
Because the head that reads the HDD floats on a micro cushion of air generated by it's aerodynamic properties. It uses this cushion in order to minimize the friction along the surface of the plate in the HDD. If you open the case, dust gets in and damages this cushion.

If the sealed case is opened, dust can get in. If the dust is gets on the spinning platter and hits the read/write head, the head will be pushed away from the spinning surface, than it will bounce back and hit the spinning platter damaging the surface and any data on it. This is called a head crash or disk crash and make the card drive useless.