There are several different reasons that could make it necessary to open the case of a laptop, notebook or PDA.
repair broken hardware
get some hardware info, which isn't available otherwise, e.g. reading the sticker on an undetected chipset
remove the speakers (speakerektomy, as described in Visual-Bell-HOWTO )
install overdrive for CPU
reflash the BIOS
change BIOS battery
implement additional hardware, e.g. an internal wireless LAN miniPCI card
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 maintenance/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 and PDA manufacturers declare the warranty to be void if the case was opened by people other than their own staff. If you want to try it anyway you may find some interesting links about how to repair, disassemble, upgrade or mod laptops or notebooks, dissect, repair and upgrade broken PDAs and HandHelds, as well as take apart, repair and upgrade mobile (cell) phones, open, repair and upgrade mobile audio and video players and repair and upgrade printers.