Google loves animations when it comes to the most recent versions of Android, and while most of the people on Lollipop can see this in various places of the system UI there are also plenty of apps which have implemented this element of the material design inspiration. Google's own Play Store app is one place where even users not on Lollipop will see some animation taking place, but the most recent version which has started to roll out to users is adding a little bit more to the mix with the return of the hamburger icon to arrow transformation.
This simple but enjoyable little transition animation should begin showing up for users whenever they move between the hamburger menu and back to the page which they're viewing. It also shows up when users are browsing any given category like apps or games, or like movies as seen in the image here. For example, if you're browsing a list of movies in the movies section of the Play Store and then you tap on a movie you want to know more about, the hamburger icon will turn into an arrow and then back into the hamburger icon when you go back to browsing the list of movies.
It's a small and subtle addition to an already great app, and it's purely aesthetic, but these things can sometimes be just as nearly important as the actual functionality of the app. First and foremost the app has to work well, but the beautification of things within the Android OS even down to the most minute details like these app animations can make a difference for how much some users enjoy their experience with certain things. These little tweaks are just starting to roll out to the Play Store app, so if you're not seeing them right now chances are it hasn't rolled out to you yet and it could still be a little while before things pop up. Since this isn't adding any new functionality to the app a little bit of a wait shouldn't be a huge issue for most people.