Most models have built-in amplification. As a result, they don't require a separate home theater receiver -- you just connect your audio components, and you're all set.
Some virtual surround and Left-Right-Center sound bars don't have built-in amplification. You need to connect these passive models to a separate home theater receiver.
Passive sound bars are a good option if you already have a home theater receiver you'd like to use. Plus, a separate receiver can provide cleaner power and more sophisticated audio processing, so you might enjoy higher-quality sound.
So like every other question, the answer is it all depends.