Players start the game with two Engineers. This provides them with a thin layer of defense and also allows them to make up to two Drones per turn.
In the early game, Engineers are useful for providing Energy for making Drones. Later in the game, Engineers are good for providing granular defense.
Buying Engineers for Energy
On turn two, buying a third Engineer in order to make three Drones per turn is common. When playing a third Engineer opening, it's important to buy three Drones per turn for at least two turns after going for third Engineer.
Going up to four or more Engineers in the opening is usually incorrect. The main time four Engineer builds are correct is when an energy-intensive unit like Vivid Drone or Galvani Drone is in the set.
Buying Engineers for Granular Defense
Having a pair of Engineers around for defense is the most common way for a player to keep their defense granular. Engineers are non-prompt, so if Engineers are going to die on defense, a pair of Engineers should be made ahead of time to replace them.
Buying Engineers as a Gold Sink
Even if granularity is not a concern, if you have extra gold in the mid or late game but not enough for extra tech to be a good idea, Engineer is usually the right purchase. When compared to a Drone that gets used for defense immediately (without being clicked even once), the Engineer can block just as well but costs 1 less.
Spending Energy in the mid to late game
Usually, the only way to spend Energy is on economic units like Drone. Once players are in the mid game, players no longer want to make economic units, and so Energy becomes useless. Most of the time, players simply allow the Energy to decay at the end of their turn. However, some advanced units like Galvani Drone, Electrovore, and Cauterizer allow players to spend their Energy instead of letting it go to waste. This is valuable, because players want to keep a pair of Engineers around for defense anyway.