DMM or Digital Multi Meters is a must have tool for the hobbyist vaper. I say hobbyist is because they are mostly used for checking ohms of a coil when you VV or VW mod can't read the coil. I am pretty new myself, 5 weeks vaping, but I got tired of cartos, clearos and pretty much anything that requires expensive, $1.00 or more, consumables. Yes, I know some can be rebuilt but those are a hassle and they were never designed to be rebuilt.
I like the Kline Tools MM200. It has a lot of capability that the inexpensive ones simply do not have. To name a few Auto Ranging, Automatically removing lead resistance, Temperature, Continuity, and a backlit display. There are many other features that make it worth the price but those are the most important. You do not NEED this DMM but is a great bang for your buck to help keep you safe. It is also usable for many other things around the house and in your car.
If you starting doing RBAs, Mechanical Mods and Rebuilding Disposable heads some form of reading device is necessary. I would not trust a mod to show accurate readings time and time again.