The Sims series will always have expansion packs. It's part of the experience to slowly add more content to the game. Usually these packs add active content like new town locations, new interactions, or new career paths for your sims. Seasons adds a little bit of this stuff, but it's mostly a passive expansion in that it adds things to the background of the sims' lives.
Seasons mostly does what you would expect. It adds seasons, weather, and holidays to the game. While it's neat to have these features, I'll just come out and say that it's not worth forty bucks.