I know patent trolls are about as popular here as child molesters, but here I am, coming to their defense..,
Suppose you are the inventor of something marvelous, like say, intermittent windshield wipers. You are not likely to have the capital to start your own car company, so how do you monetize your invention? You do the obvious: approach the existing car companies about licensing. Now, if you don't happen to know the story of Rober Kearns, you may want to look him up, but the TL;DR version is that if you are not ready to spend years and $MILLIONS in court, the giants will just steam roll right over you, taking your invention with them.
Enter the "patent troll".
Patent trolls are your key to monetizing your invention. They have the expertise and the money to see a court case through. They are not producers themselves so the multi-nationals can't shut them down using their own patent portfolios. If the patent is a good one, they stand a real chance of winning in court and they compete against each other for such opportunities, so they form an alternative market where your invention can fetch you a tidy sum. They will expect a discount obviously; they assume a substantial risk, after all, due to the uncertain nature of litigation.
The facts that patent trolls don't invent anything and don't make anything are often held up here on Slashdot as reasons to deride these companies. These are red herrings. Many companies exist which perform valuable functions in society without doing either of these things. Patent trolls are among them.
I will grant that there have been some absurd patent cases ltigated by patent trolls, but that's a separate issue. If anybody's reputation should suffer for these absurdities, it should be the patent office's. The troll is just doing its duty by its investors to run a profitable company by obtaining maximum value for its patent assests.