It is necessary to remove the top cover if you need to replace the laptop touch pad, witch is attached to the top cover, or replace the motherboard.
Steps 1-2 shows how you can access laptop memory, hard drive, wireless card and modem.
Steps 5 explains how to remove the DVD drive.
Steps 7-10 will be useful for removing and replacing the keyboard.

The motherboard removal instructions will be very similar to instructions for a Toshiba Satellite M105.