There are several different reasons that could make it necessary to open the case of a laptop.
repairing broken hardware
get some hardware info, which isn't available otherwise
remove the speakers (speakerrektomie, as described in Visual-Bell-mini-HOWTO)
install overdrive for CPU
change BIOS battery
Repairing a laptop can be quite expensive if you don't have a manufacturer's warranty. Sometimes professional support is bad. But opening a laptop case can be difficult. Often the procedures to upgrade the memory and the harddisk are described in the manual. For further details, you should try to get the maintainance/technical manual. Just be extremely careful and make notes as to where each screw goes. You must get most of them back in the right hole or you could ruin the machine by damaging the system board. Also after you get all the screws to an assembly out (some will be hidden) the parts are usually held together with plastic clips molded in, so you still must exercise care to separate them. Sometimes you need certain tools, for instance TORX screw drivers or a solder kit. Good luck.
Usually laptop manufacturers declare the warranty to be void if the case was opened by people other than their own staff.