Yes, battery tenders do work! But DO NOT TRUST the cheap Harbor Freight ones or equivalents. Spend a little more & get a quality unit. We had the unfortunate experience of 3 batteries burning off from overcharge with the cheapos. One battery was salvageable, the other 2 needed replacement.
Batteries are sensitive to many parameters. If managed well, they'll live a long life. If not, their service life & discharge depth drops off quickly.
Battery tender or not, it's a good idea to start a vehicle at minimum once per month & run it for 12-15 minutes. If equipped with A/C (R-12 or R-134A systems alike), run that too for the same period to keep the seals up & oil circulated. Saves having to replace a seized compressor later, and keeps you from leaking out freon.