Hi,
If you're looking to save space on your phone (I did as I have the HTC 8S), you can easily do that by installing your apps from your SD card.
On the Windows Phone store website, scroll down the app page you want until you see the Download and install manually link. It's at the bottom of the left column.
Right click on the link to save to a folder on your PC.
Plug your phone in and using File Explorer, move the .xap file you downloaded to the root folder of your SD card.
Unplug and reboot your phone.
Open the Store app on your phone and click on SD Card.
Wait while it populates. Your app should appear. Click on install.
It takes a few minutes sometimes for the app to appear under SD Card in the Store.
I find if I reboot it helps.
Good luck.