Why is there an option to link my Facebook, LinkedIn or Google account?

For the exact same reason that you can verify a phone number or email address, you can also link to your social media accounts. When linking with Facebook and Linkedin it also shows how many connections/friends you have, confirming the authenticity of your profile. In addition, it is very useful to make a link, because you can then log in directly to your Gearbooker account via that account.