Why do I need to use a power amplifier after using a voltage amplifier?
However, couldn't I use a single voltage amplifier which gives me the desired voltage to drive the load, why is there the need to have two separate amplifiers? 


You can, but you might find that internally it uses two stages of amplification. If you read up on gain-bandwidth product you will find that this figure limits the maximum gain of an amplifier for a given bandwidth. The workaround for this limitation is to amplify in two or more stages.

Distortion will generally be lower with lower gain as well.


  And also why is it called a power amplifier as opposed to just a voltage/current amplifier?


The 'signal' of signal amplifier indicates that we are dealing with signal levels typically of 1 V or less and low currents in the range of a few mA or less. If you look into a power amplifier you will find power components such as "power transistors" and a suitable power supply. These are required to drive low-impedance loads such as loudspeakers. The terms are old but have a fairly clear meaning for an audio engineer.