Leaving a carcass in the wild would attract scavengers and spread disease, which is prevented by inhumation. Cremation is also an option but requires a large amount of fuel. In early and prehistoric cultures, inhumation was the easy option.
Edit: to add to this, I read something on how the first “burials” were just piling rocks on top of the dead body, primarily to ward off scavengers. So, aside from honoring the dead, burial and cremation have a practical purpose as well.