Phone numbers are less disposable than email addresses (generally speaking), and thus you are less likely to have fraudulent/shady transactions occur on your site if the user is phone verified.
This may not be super important if all you want to do is prevent lowly Forum spam (ReCaptcha may be all you need in this case). But phone verification can be particularly useful when the cost of fraudulent transaction is high (like user purchasing goods on your site with a stolen credit card, etc).
As an aside, I saw Amazon and Craigslist now use phone verification for more critical parts of workflow. Which inspired me to build Phoneify as a JS widget than anyone could use.
