In "normal" mode, selections can be made with the keyboard or mouse in several ways, the most obvious being holding shift while pressing the arrow keys.
When entering a formula, these same functions do not select but enter the would-be selection into a formula as a range.
Except when selecting an entire row or column. Assuming cell B2 is active:
-Shift+space (usually selects the entire row) should insert "2:2" into the formula, but it inserts a space.
-Ctrl+shift+space (usually selects the entire column) should insert "B:B" into the formula, but it makes a "normal" selection.
-Clicking the column or row headers should also insert appropriately into the formula, but do nothing.
Shift+Space → Entire row
Ctrl+Space → Entire column
Ctrl+Shift+Space → Entire sheet
This bug may relate to:
Bug 92452 - Keyboard to select entire row or column or sheet when in formulae mode
Bug 92453 - Entire column functionality in formulae keyboard shortcuts
Bug 96981 - All cells selector button not functional when in formula mode
Shift+Space, Ctrl+Space, Ctrl+Shift+Space, and clicking row/column/all headers do not work as expected when inserting into a formula
*** This bug has been marked as a duplicate of bug 92452 ***