Pixie

 is a cheap unit  used to gain offensive finesse and increase one's damage threat.

Strategy
A Pixie is sacrificed for Attack when you click it. Assuming a blue costs more than one gold, then at a Pixie costs more than the Engineer that it will likely kill. Buying Pixies for damage is therefore inefficient.

The main use of Pixie is to spend that you otherwise would have a hard time spending. In the early game you can buy a Blastforge to get a Wall without having to either let the decay or spend a large amount of  on a Steelsplitter on subsequent turns. You can instead buy a Pixie or two while spending the extra on other units. Pixies also combine well with high tech blue units that don't use all your  every turn after they are bought such as Odin, Apollo, Defense Grid, and Energy Matrix by cheaply spending extra.

The damage that Pixies have is very flexible which punishes a lack of defensive granularity. By choosing how many Pixies you want to click you can force your opponent to defend in an uncomfortable way.

Stacking Pixies
A common beginner strategic mistake is to constantly use every turn to create a large stack of Pixies, hoping to eventually Breach. This strategy isn't valid because all it takes to counter it is the opponent buying an Engineer for every Pixie and just defend the rest of the constant damage without using it, making it so when the Pixie stack is detonated each Pixie kills an Engineer and nothing more.