There are many systems of the vehicle that only operate when the car is at speed and under load. This cannot be replicated while the vehicle is stationary or moving slowly in a parking lot.
Engines make noises. Some of them are normal, and some of them are really bad. If you only hear a whistling sound under full throttle conditions, but cannot determine where it is coming from, you can't exactly have a buddy hang out the window and listen while shooting down the highway at 80mph. The dyno allows us to be near the engine under varying speeds and loads.
The dyno is a measurement tool. If you want to make more power, don’t you want to also know where you are at before you add components? We can determine if an engine is healthy or not based on it’s power output and running conditions. A vehicle that is dropping power between runs may have an engine that is failing or possibly brakes that are sticking.
We have thermal imaging devices to pinpoint and differentiate these issues. We can even use the dyno to simulate towing a heavy trailer up a very steep hill. If the engine runs properly under these conditions, it will run flawlessly on the street, and the chances of it lasting increase exponentially.
All dynos are not created equal and the facility they are located in plays a major role.
A combustion engine creates many noxious gasses. Most come out of the tailpipe and can kill a person if allowed to build up. Some are byproducts as coatings and materials become overheated. Neither should be allowed to linger in the shop. Because of this, it is imperative that dyno be located in a room separate from the rest of the building.
Making good business decisions about making power starts with knowledge, money and choosing parts, fabrication and a good shop to execute actions that lead to reliable power.