Because it is the main circuit board in the computer that connect to all the other boards and component.
A system board or a mainboard can be called a motherboard, or "mobo" for short.
This is commonly called a "Mother board"A MOTHERBOARD OR A MAINBOARD
Motherboard/Mainboard
It is usually referred to as the "motherboard." Less common terms include "mainboard", "logic board", and "backplane."
The motherboard aka mainboard
A motherboard (also called mainboard, main circuit board, or mobo) is the main printed circuit board (PCB) in general-purpose computers and other expandable systems. ... Motherboard means specifically a PCB with expansion capabilities.
Because you might drop the drives on your motherboard while installing.
Without the mainboard, also called motherboard, your computer would not work... at all. Everything is hooked up to it from RAM to hard drive to CPU. The only hardware component it doesn't control is the power supply, but the power supply is also hooked up to the motherboard.
Planarboard is a other word for motherboard or mainboard. This comes from the company IBM/Lenovo. If you look in a bios of this machines, you find in the settings this "planar" word, e.g. planar-ethernet oder planar-controllers. Apple's mainboard called by logicboard. Conclusion: A planarboard/logicboard/mainboard or sometimes mainbase is the same peace of hardware, the mainpart of a computer or similar machine.
Another term for motherboard is mainboard.
The motherboard, or "mainboard" of a system is given away by the name. It is the main component to a PC, as everything else tends to connect to it. for a more detailed explaination, check out this link: http://en.wikipedia.org/wiki/Motherboard
Yes, some manufactures (mostly OEM) call use "system board" definition instead of "motherboard" (custom builds).