I have found heating mats improve consistency of germination / sprout times. A much larger portion of my plants sprout in a smaller frame of time. I grow in cold months, so it makes a big difference for substrate temperature. I don't use it after seedling stage or once a cutting is well-rooted. It's helpful when paired witha dome to create a micro-climate for plants that need higher RH%, too. Much cheaper than trying to control climate of entire room, and if indoors, unusually high RH is bad for your house anyway.
If the substrate is ~76F on its own, you won't need one. Also, you don't continue to use it beyond those early stages.
I am very glad i finally bought one. The consistency is useful. Do get one with a thermostate probe, otherwise it's a crapshoot with the temperature. If you live in a warm climate, may not gain any benefit from it.