Rob from Twilio here - thanks for using the service. Localization and fraud/spam protection are definitely the two most useful for Twilio Lookup, but suspect there are a couple more than might be less obvious.

1) Smarter Routing - quite a few developers have expressed they'd like to know whether or not a phone number is a landline or not before attempting to send a text message. Twilio Lookup will describe whether the number is mobile, landline, or voip so you can choose the appropriate communication to reach the number.

2) Error Checking - While libraries like libphonenumber are useful for determining if a number can possibly exist, there are some conditions where determining if a number does exist is more valuable. This can be used for error checking - if I as a user input a typo in my phone number (e.g. I mistype my personal phone number as '(347) 193-6073' which does not exist instead of '(347) 923-6073')

I also suspect across larger applications carrier info might also contain useful demography. For example, knowing that your users in Montana mostly use Verizon might inform your advertising to surface more CDMA devices in that market.

Ultimately this will probably end up like every other product we release - the killer use case is one we never even considered.