Each type has its own benefits and may be used for different purposes.
Selecting rectangular regions
Selecting rectangular areas may be useful if you need to crop an image to a certain size or, for example, if you want to fill a rectangular area with color.
To select a rectangular region, choose the corresponding button from the toolbar window, click and hold your left mouse button in the editing window, dragging it across the area you would like to select. When the area is covered, release the left mouse button. The region is selected and you can proceed to work with it.
If you need to select a region that is larger than the area displayed in the editing window, you can use the arrow keys to aid you while applying the selection tool.
Note that to select a rectangular area that covers all the image you should use the "Select all" button on the main toolbar.
To select multiple regions, hold the "Shift" key down as you select the second and all the next areas. You can then work with all selected areas simultaneously.