There are several reasons that you should measure the power output of your laser.
1. To be able to get consistent results when working with your laser. 100% power on a brand new tube will not be the same as 100% power on a 3-year old tube. If you know what the actual power output is, you can make better choices when selecting power settings in your projects.
2. Along the lines of reason 1, knowing the actual power output can help you get consistent results when working with different machines. If you have multiple machines, or ever work at a maker space or with a friend, you can be sure you are applying the same power if you measure and calibrate your output.
3. To ensure you get what you pay for. If you buy a 100W tube, you may want to make sure you are seeing around 100W of output. Without a power probe, you just don't know.
4. It's very useful for troubleshooting. If you aren't seeing the power output you expect, you can quickly determine if it's the tube or the optics by measuring the power output directly from the tube. It can also help you know when the tube has reached the end of its life.