If you have set up icloud photostream or other icloud features, you can also download apps automatically to any of your devices.
Have both an iphone and ipad? Set up the “store” and download to both devices. Doesn’t matter if you purchased (or free download) from itunes on your PC or Mac or on your other idevice.
Go to settings on your idevice and make sure Music, Apps and Books are turned to the “On” position. In most cases leave the “use cellular” data off. Or you could very quickly use up your data allotment. In the off position it will only download when you are on wifi.
So if an app is works on both iphone and ipad, it will show up on both devices with no more effort on your part. You must be set up to use icloud for this to work.