Why we need another kind of memory: RAM inside the computer if we already have hard drives?

It’s because the speed with which Microprocessor (CPU) communicates with the RAM is much faster compared to the speed with which CPU interacts with the hard drive.  RAM is also a kind of microprocessor so it interacts with another Microprocessor (CPU) quickly and effectively.

                                                 

If you want more information about this go through the link below:

http://www.salmanashrafny.com/2011/03/what-is-ram-and-why-do-you-need-it.html