They are also typically used as a "mutually assured destruction"[1] war chest. For companies that have no intention of filing a patent lawsuit, but need some kind of insurance against that happening to them.

You file, buy, acquire, etc, a large enough portfolio of patents such that if another company sues you, your broad portfolio almost assures they are "violating" one of your patents. Now you have leverage to make the initial lawsuit go away.

[1]https://en.wikipedia.org/wiki/Mutual_assured_destruction