You can’t download Google Play apps to your computer. The apps you see in the Play Store are written to run on devices that use the Android operating system.  Your computer probably runs either the Windows or Mac OSX operating system.  They aren’t compatible. That’s like trying to buy a game for the Wii U and play it on your PS4. Some developers do make PC or web versions of their offerings that can be run on computers.  Check their website.  You can find it at the bottom of the app’s page in the Play Store.
 
You can use your computer to read books, listen to music, and watch movies from the web version of the Google Play Store.  You just can’t use the apps.