This is strongly recommended, there are things that can go wrong with your UPS and they require periodic maintenance. As with all support contracts; you’re generally spending a little extra money to reduce risk.
The batteries will periodically need to be replaced. This will happen more frequently the more (and deeper) the batteries are cycled.
Replacing the batteries every battery that has been in continuous service for five years probably should be considered suspect until proven otherwise.
Like any other electronic device, a UPS can fail. You need to have a plan for this.