Keeping the environment in optional range is very important to optimize your yield at harvest.  There are quite a few reasons why you might want control the speed of your fan.  If you plan to upgrade your set up it is a good idea to get a bigger inline fan than you need, and lower the speed to the level you need.  Then once you step up your garden you do not have to go out and buy a new fan.  If it is too hot or cold outside you can slow it down to still allow some circulation, without having to work as hard keeping your temperature and humidity inline.

I bought mine because the flow of air coming out the window made a lot of noise, It also came in handy to help control the temp with both hot days and cold nights.  My room would heat up during the day when the lights where off.  This caused me to have plant night temperature higher then their day temps.  That added a lot of unneeded stress on my ladies and was the worst harvest I had in a while.  I got it dialed in so that once the temp  started raising I would drop the speed to about half.  Allowing me to keep flow in the room, while not bringing in so much hot air.  During the winter months the opposite is true.  

I bought a controller manufactured for Oracle Garden Supply.  I say "manufactured for" because I have seen at least 4 companies selling the same re-branded product.  Most popular being the speedster.  I picked mine up on Ebay for $22.00 shipped, and worth every penny.  All you have to do to set it up is plug you fan into the controller, and plug the contoller into a 120v outlet.  The controller has 3 options.  Full, off, and variable.  When variable is selected it will pulse power to slow down the current going to the fan.  

About the only downside of this and most all speed controllers is that the fan will hum when variable is selected.  Depending on how slow you have it a lot of times the fan is louder than the hum.  Also with the power pulsing on and off it can cause the fan to run warmer than usual and if ran to slow the motor could burn out sooner than if ran at full speed.  I am currently collecting the supplies I need to build a DIY fan controller that will use a variac.  Using a variac will eliminate both the heat build up and the hum created by the fan because rather than pulsing the power it will actually lower the voltage going into the fan.  This is essentially what the Can-trollers are.