Garuda Purana is one of the most sacred Mahapuranas for the devout Hindus. It gains importance because of a single factor : It is the only PUrana which gives a detailed description of the postmortem conditions and rituals as believed by this faith. However, it is a belief as spread by the dogmatic priestly class.
The fact is that the Garuda Purana contains many more details other than these. It reveals the consequences of an action in a very graphic and logical way. While most of the puranas tell one what to do, it also tells us what one should not do.