The sms:// share app will auto-populate a SMS message depending on the format on the sms:// link:
- sms://+12345, 54321 - message with multiple recipients
- sms://+12345?body=Message text - message with pre-defined content
If there's no content in the sms:// link a new SMS message is invoked.
While it's certainly not an essential app it does fill a Windows Phone functionality hole. The app uses a new feature in Windows Phone 8 that allows app to register themselves as being able to handle a certain protocol (sms:// or sms: in this case). There are a number of other app that also "take-over" this SMS link format (e.g. NFCsms). If more than one app is installed Windows Phone will ask the user which app ti use.
Registering such a link is referred to a "registering for a URI association" in the Windows Phone developer documentation:
To handle a URI association, specify the corresponding URI scheme name in the app manifest file, WMAppManifest.xml. Your URI scheme name can be any combination of letters and numbers; however, any URI scheme names that are reserved by built-in apps will be ignored. For more information, see Reserved file and URI associations for Windows Phone 8.
Once an app has been registered for an association it must "listen" for a URI and take appropriate action when it is automatically launched.
You might wonder what happens if sms:// share (or another app using the association) is not installed. Here's the official answer from Microsoft:
When the user launches a file or URI from an app, what happens next depends on which apps are installed on the phone. If no apps on the phone can handle that particular file or URI association, the user will be given the option to get one that does.
When there is only one app on the phone registered for a particular file or URI association, that app will be automatically launched when the user wants to open it. If a user has more than one app on their phone registered for a file or URI association, each time the user opens the file, they will be as asked which app they want to use.
sms:// share can be downloaded from the Windows Phone Store. It costs £0.79 / $0.99 / €0.99 and there is currently no trial available.