Why memory is essential for computer?

It is very important. When a Computer starts up, the Operating System gets loaded into the Memory. Once every while, the System checks the Memory to quickly access certain programs, only if that program has been loaded into Memory.

Buying Memory for your Computer also increases the speed at which your system may perform, or rather, load programs. More Memory means more space for your system to process with. [NOTE: Not talking about Storage Space, but system cache.]