Some apps that are installed on your PC might not be pinned to Start. But you can always pin them if you want them there. On the Start screen, open the Search charm, select the app you want to pin to open its commands, and then tap or click Pin to Start.
And if there are apps you don't use, you can unpin them from Start. On the Start screen, select an app to open its commands, and then tap or click Unpin from Start. If you unpin an app, it's still installed on your PC, so you can always find it again with the Search charm.