They are building a mathematical model of you. By making you sign in they can be sure it's really you.
You can tell where the industry is going by the types of popups Google gives you. For the last year or two it's always been 'sign in for the best experience'. The little guys, Disney is a little guy here believe it or not, copy the big boys.
In the past it was 'add your cell phone', and/or download the app which knows your cell number and can connect the dots. They are cross correlating all these methods to identify you, and model you.