A career-based expansion pack and introduces new career pathways, changing the style of gameplay involved in the game. In the non-Ambitions version of The Sims 3, all careers were known as "RabbitHoles", a reference to Alice in Wonderland and her disappearing in a rabbit hole. All careers would have a specific building (Political : City Hall; Law Enforcement : Police Station; Medical : Hospital; etc). However, with Ambitions, the new careers have completely hands-on gameplay, meaning that the player can control whatever their sims do in their careers.
It's a nice expansion for those who focus more on the careers, but I don't really think that is a 'needed' expansion, there are other that, in my opinion, are much more fun!
Not bad of an expansion, but all I have to say is you can play The Sims 3 without it. There really isn't much to miss out on unless you actually want to control your Sim while at work.