Or sometimes their software licences apparently: http://www.businessinsider.com/why-marissa-mayer-bought-a-30...

A company can be bought for anything really, if it makes sense.

As shubb points out, if it's cheaper to buy a company than to deal with licensing or legal problems, then it might be bought. Same if the overhead of dealing with a company as an external entity is too high compared to buying it outright and forcing it to come in-house (if the product or service isn't of any need after some time, the employees can still be shuffled in the org afterward)