A simple machine with a large mechanical advantage means you have to put in less input force to do the work that needs to be done.  Look at the mechanical advantage as a "multiplier" of the force you are putting in.  The force you are inputting is multiplied in some form or fashion, depending upon the transfer of the force within the simple machine, by the length, number of wheels, or gear ratio within the simple machine.

A simple machine with a large mechanical advantage is able to do the work you have selected it for with more ease.  Generally, the better a machine does it's work causes the price of the machine to increase.