Of course you could use your phone for gaming as well as other things, but a lot of apps can begin to clutter your phone up, and you only have so much room and memory. Separating your gaming from your normal everyday use can really help organize your phone usage, and your life.
If you like to play games on your phone, you may want to install your games on your old Windows phone only, especially if you find yourself only gaming on your phone at specific times of the day such as at night before bed or while waiting somewhere during a daily routine.
