Watzrcooling reservoir

[The321]

Why do most water cooling setups use a reservoir? 

[Torhus]

In theory you dont need one, but there are different benefits.

Some do it for looks, some have it for ease of use (easier fill and avoidance of air)

 

It should also help lower temps, as more water, means more room for the heat to expand out to

[Christophe Corazza]

You don't need a reservoir, but they may help reduce temps. Depending on how many components you're cooling will depend on whether or not it'd be a good idea.

Reservoirs are really useful for bleeding air out of your system but their real purpose is to act as a kind of "capacitor" to damp heat spikes by adding more mass to absorb heat. 
However, having a huge reservoir won't lower overall temperatures. Once your system gets to its operating temperature it doesn't matter how much mass you have in the system it will be at that temperature. The extra mass just means the system responds slower to temperature changes (that's good).

[RollinLower]

Also, when doing complex loops a res is pretty much required, as you need something to feed the pumps when you're filling the loop. Also, in complex loops completely bleeding the system for air might take weeks, and a res functions as a place for the excess air to collect.