Regardless of whether you calculate or guess on amounts, how accurately you spread your mulch will make a difference. You can goof up on spreading mulch either way, but you’re more likely to goof up on getting too much or too little if you just guess on what you need… that is unless if you don’t calculate things correctly.
Another reason why you may want to learn these calculations is if you get into landscape construction, rather than just maintenance. Some states require that you to take an exam and to be licensed to do this work, and if your state has an exam like the one here, there could be a number of questions on the test where you have to figure out how much material is needed. They give you a landscape plan and you have to go from there. Guessing won’t work.