The shortcuts on computers exist because you have (at least) two different ways of interacting with your user interface, namely using a keyboard, and using a mouse / touch-pad. In order to switch from one to the other, you have to move your hand(s) to use the other method. Also the mouse requires precision to use correctly, something which may be difficult for some people. Users may — if they are familiar with the shortcuts — use the computer more efficiently by using shortcuts.
The typical example would be using TAB to step through the elements of a form, something which requires a simple button-press when using shortcuts, but that requires lifting one hand, moving mouse pointer, stopping mouse pointer at the right place, clicking, moving hand back to keyboard when using the mouse.
On a smartphone with touchscreen, you have only one method of interaction: poking at your screen. You never switch back and forth between different methods of input. If anything it would rather make the interface confusing and harder to navigate if every you needed to call up a QWERTY keyboard on the screen to access application.
Note that you can make shortcuts on a smartphone. This is especially prominent on Android phones where your point of entry of the Home screen, where you have shortcuts, widgets and similar that provide quick access to different functions. And then you have the "Apps" listing, which shows everything.
You also typically have a "search" function (rarely used it seems) where you can access all apps and content directly from the Home Screen after typing just a few letters.