While the main reason that you would subscribe to PlayStation Plus is to be able to compete and play with players online, an added benefit of subscribing is that you're gifted a couple of free games each and every month of the year.
For instance, January's free PlayStation Plus games were
Uncharted: The Nathan Drake Collection and
Goat Simulator —
find out more about those here. As of today, those titles are no longer available for free but you can still get your hands on them for their usual prices.
Those two games have been replaced by February's free PlayStation Plus games — those being
BioShock: The Collection,
The Sims 4, and
Firewall Zero Hour. This trio is available for free to PlayStation Plus subscribers as of today and will remain free until March 2nd. The latter of those is pretty niche in its appeal, being a PlayStation VR game.
BioShock: The Collection consists of all three
BioShock games —
BioShock,
BioShock 2, and
BioShock Infinite. They're acclaimed, retrofuturistic single-player first-person shooters.
The Sims 4 is a simulation game in which you essentially play God and do your best guiding your Sim character through their everday life.