When you want to edit the highlighted cell, and you don't want to move back to the previous cell when pressing the <- key, you press F2.
Pressing F2 ensures that when you're editing the cell, the cursor will move back to the previous character and not the previous cell.
In Google Docs Spreadsheets?
In Google Docs spreadsheets, the only way to achieve similar functionality to F2 in Excel is by double-clicking the cell with the mouse.
Pressing enter on the selected cell in Google Docs re-instantiates the behavior where pressing the <- key moves you to the previous cell.
Is there a keyboard shortcut in Google Docs Spreadsheets that creates the same behavior as the Excel F2 key?