Because the screen is small you just don’t have room to have a control or button for every type of action. Hence it is sometimes necessary to rely on certain behaviours that Apple has built into the iOS user interface.

Some examples of these are:

1) Pinch to zoom out – if a view is zoomable and not already zoomed out to full extent then move fingers together to zoom out

2) Reverse-pinch to zoom in – if a view is zoomable and not already zoomed in to full extent then move fingers apart to zoom in