The Expand Selection command in the Block menu is only useful when selections are persistent. Persistent selections allow you to move the text cursor away from the end of the selection. The Expand Selection command will create a new selection that includes the old selection, and all text between the old selection and the position of the text cursor. The position of the text cursor will be the new ending point of the selection.
The most significant difference between Block|End Selection and Block|Expand Selection is that End Selection will pivot the selection if you've moved the text cursor above the spot where you started making the selection while you ended the selection below the starting position, or vice versa. This causes the old selection to be completely deselected, as the starting position stays put. Only the text between the old starting position and the new ending position is selection. Expand Selection will always expand the selection. If the new ending position is above the old starting position while the old ending position was below it, or vice versa, Expand Selection simply makes the old ending position the new starting position.
Whether selections are persistent or not, you can achieve the same effect by pressing Shift on the keyboard while clicking with the mouse at the position where the new selection should end. The advantage of Block|Expand Selection is that you can assign it a keyboard shortcut, and expand the selection with the keyboard.
Block|Between Matching Brackets
Block|Go to Beginning
Block|Go to End