Pfffft, take a lot to make cry. Some Lion King and death are about it...okay, maybe salted wounds, too.
With that said, a hydrometer is extremely. Basically you take a gravity reading pre fermentation and another reading post fermentation, toss that info into an online calculator and you have your ABV. That simple...
In addition when you start brewing more complicated beers a hydrometer will become extremely valuable. For example if you ever get into traditional sours you need to pay extreme attention to hydrometer readings to know when your beer is ready to bottle, if you are bottling. I assume there are a billion other ways you can use it, but for me it's primarily to determine the alcohol level as a percentage and to know that I'm not about to throw glass all around the garage...or someone else's garage.
In addition to the hydrometer make sure it either comes with or that you buy a vial or tube for it to sit in because it doesn't do any good without that.