2 games per platform a month, or 72 games a year, and free access to all "claimed" games. Which is at least 24 games per platform but many are cross buy, Vita especially gets almost anything cross buy.
Automatic uploads for save data to the cloud (1GB of storage--usually more than enough)
Online play on PS4 (it's free on all other platforms)
Assorted discounts on PSN games, usually either at launch/preorder or during certain sales. Often roughly an extra 25% off
There's also some manner of auto downloads, I think PS3 only and on PS4 that's a free feature. And some exclusive Betas/demos but that's a pretty minor value-add IMO. If somehow none of "free games" "discounts" or "actually required for online play on PS4" grab you I'm sure you can live without it. IMO it's way worth it for the free games and discounts, and the online play requirement is just an unfortunate reality.