The move obviously is to solve the problem of users having to screenshot or download images directly to their smartphone’s storage. Explained an announcement on the official Google search blog:
Let’s say you’re searching for “bob hairstyles” on Google and an image catches your eye. Simply select it and tap the star. Next time you’re at the stylist, you can easily access the picture without having to dig around or do another search. Once you’ve starred a few images, you can keep them organized in folders: to add an image to a folder of similar items, tap the pencil shaped edit icon. Create a grouping such as “haircuts for the winter” or “snowman ideas” and your image will be added to a folder with similar ones.
Image Credit: Google blog