This was taken from cleatech site
Dramatically improves flow control and reliability of your your system. Does away with nose pressure switches, and manual flow systems - the 2 biggest causes of pump failure on WFP systems.
To be honest the price is not a lot differant than a new pump.
Now if your pump is lasting 3 years i would not worry about one.
I think it is just another thing that could cause problems.
I have a varistream i bought it about 18 months ago and used it for about 3 months and i didn't really get on with it.
I also tried it on my van mount and again didn't like it.
But thats me some people swear by them
Keep it simple less to go wrong.
Paul