Right - and the point is that in a casual email you may reply 'yes that's fine' and is that signing a contract or not? But if we have some ceremony, like a physical signature, or using this // notation, then it's clear what your intent was.
Even a verbal contract has some weight. A "yes that's fine" reply to an unambiguous email is likely to stand up in court. "I, <full legal name> agree to this" is even better.
A formal contract with a real ink signature makes things easier, but it's often not required.
