They are super useful for measuring your consumption. We use them all the time, when calibrating drivers. Especially with Mean Well drivers, which will let you set them above their recommended rating, and with constant voltage drivers in general, which can have exponential IV curves, wattmeters can lengthen the lifetime of your lights by years.
With the kWh counting feature, you can get a precise "gram per watt" measurement at the end of your grow.
When lights have PAR maps, these inexpensive meters can give you all the information you need, to work out the PAR your plants are getting, at given distances.
Standard 16A max, 230V input, with +- 2% accuracy. (Up to about 3600W)