If YOU have an irrigation system for your plants, then you do not need a water gauge. The water is precisely portioned by the pump but especially by the timer. As long as the soil is moist, your plants are well served. However, a water gauge is very useful in a double-bottomed system.
With watering cones and similar variants, the water indicator is already present. You can see how full the containers are, so you can easily estimate when you need to refill with new water. You should also make sure that water reservoirs always contain water.