Lots! Reservoirs serve quite a few basic functions.

Since you are increasing the total volume of fluid in the shock, the fluid will stay cooler than a shock that does not have a remote reservoir.  The cooler fluid serves two functions.

Since heat destroys things like seals and such, your shocks will last much longer.  All shocks either wear out or need to be serviced from time to time.  With cooler fluid temps over longer periods of time, your service intervals will be greatly extended.  And when the time to rebuild your shocks comes around, the price for parts will be reduced as well!  You could consider reservoir shocks to be less expensive in the long run.

Also, as fluid heats up, it changes properties.  Consider the syrup you pour over your pancakes!  The fluid inside your shocks reacts the same way.  When its hot, it gets thinner in consistency, which will change the way your shock reacts as the heat in the shock builds up!

If you set up your shocks properly, you would want to *barely* bottom them out on the biggest jumps you can handle.  That way, you know you are using ALL of the suspension travel you have.  Lets say you have set your sag height all set-up properly and have adjusted your compression and rebound to just the way that you like it.  All of this being done while your riding close to your truck and your tools!  Now you take off on a nice long ride, and close to the end of your ride (now the fluid is hot), you come across a nice big  jump you just KNOW you can handle!  Can you guess what will probably happen??  You'll probably bottom your shock out a lot harder than you did before...and can't be a good thing!!!  Parts bend...wrists ache...and you wont be a happy camper for sure.  The cooler the fluid, the more consistent your ride.

Second...fluid does not compress, but the gas in the reservoir does!  The reservoir is not full of fluid.  Part of it is nitrogen gas.  The main function of gas charging is to minimize aeration of the hydraulic fluid.  The pressure of the gas compresses air bubbles in the hydraulic fluid.  This prevents the oil and air from mixing
and creating foam.  Foam affects performance because it can be compressed - fluid can not. With aeration reduced, the shock is able to react faster and more predictably. Ever have a non coil over gas shock in your hands? If you compress the shock, the shock extends on it's own. Having more gas, under higher pressure is another benefit as well. The gas creates a mild boost in spring rate. In a coil over shock that is designed
properly, the physical spring rate can be reduced allowing for a more controlled and plush ride.

Third...adjustable compression damping! This is a big plus indeed. Custom built shocks should be set up perfectly for you. But, that doesn't mean you ride the same way every day! One day your on the sand, the next day in the dirt. Having a way to adjust for different riding conditions is definite advantage.

Here is another thing that will save you on those jumps and whoops at the end of the day. A reservoir shock has much more capacity for taking on the big hits. The gas and fluid in the reservoir acts as back up shock absorption. This allows you to run your compression adjustment a little more open. You and get a much softer ride while still being able to soak up the big hits when they come! And to me...a softer ride is much more
enjoyable when your riding trails for most of the day!!!! (in the sand...this is not a big issue)

As you can see from the diagrams to the left, all shocks have fluid and gas inside them. But the reservoir designs contain a lot more. And this is one situation where more is definitely better!!! More fluid, more gas, cooler shock temps, longer life, increased predictability all combine to make your riding experience much
more enjoyable.

Basic Shock Designs

Emulsion Design

Internal Reservoir Design

Piggy Back Reservoir Design

Remote Reservoir Design