The sites have no way of adding money to your GTA 5 or GTA Online account, and the 'human verification' part of this process normally involves completing an offer or survey - which the scammers will use to harvest your details, and/or collect a commission from the offer/survey provider. These sites are scams and should be avoided at all costs.
If you've been searching online for GTA 5 money cheats then you may have come across links for GTA 5 money generators, which promise to give you free GTA 5 money if you provide them with your Rockstar Games ID and platform then complete a 'human verification' in order to prove you're not a robot.