Hydrometers don't measure alcohol - most comonly used measre eith % sugar, degrees brix or degrees balling. They can be considered equivalent. The alcohol content in wine will be approximately half or slightly more than the starting sugar content, except that yeasts can only go up to about 15-16 percent alcohol. There are really few yeasts that can go to 17-18 % . The high alcohol level actually kills the yeast. To get higher alcohol content you would have to fortify the wine.
Well I better go to the wine supply store and get my money back and return the one that I have because it's labeled to read alcohol content.
There are many reasons why a winemaker might want to use a hydrometer:
To measure the specific gravity (SG) of must or wine
To determine progress of fermentation
To estimate potential alcohol percentage at time of yeast pitching
To calculate percentage of alcohol (using "before" and "after" readings)
To measure the amount of sugar present in wine or must
To allow the winemaker to determine when fermentation is finished or should be stopped