Remote reserviors came about due to a few reasons. First off understanding how a shock works is imporatant to understanding how or why the reserviors exsist. Think of a shock as your hand moving in a pool. Close your hand and move it slowly and you have some resistance as the water has to flow around your hand. This work we call damping in a shock. Now try to push your hand faster and you will feel more damping. Do this again with you fingers spread apart and you will feel that it requires less force as the water has a shorter path to go around it. Take that analogy to a shock, you have a shaft that moves up and down and connected to the shaft is a valve with a variable hole in it. Nearly all the technolgy in a modern shock is about how to control the size of that hole which is what varies the damping rate. Some people use flexing thing metal plates other use a ball and spring while most use a combination of technics to control the hole size and therefore the damping characteristics. So who cares right, well two thing happens when you restict flow, first is you generate heat (cuz your doing work) and second if speeds are fast enough you can creat a void behind the valve which can cause damping fluxuations during repeated movements. To control these fluxuation you add pressure, the higher the pressure the less likely these voids are to happen (this is call cavitation in the damping world) and the more constant the damping will be. So most of the orginal shocks are just oil damped and with no extra pressure, then come the pressurize shocks with internal reserviors that are sealed and charged during assembly (these are the kyb gas adjust and Bilstein style shocks). Now there is the high end stuff like what Fox makes with remote resi's. Yes they are there to add more oil but they also add more volume to the resi which help resit fade and most importantly makes it easy to service. The high end stuff is all rebuildable and very tunable.
Probably more info than anyone wanted to know but my hobby is tuning dirt bike suspension (for the last 10 years) so it is a subject that I have a passion for..