Holding ctrl in the Sprite Editor allows you to manually move the pivot point with snapping. With ctrl pressed (orcmd on a Mac), you can snap the pivot to one of the pre-defined locations, as defined under Pivot on the control panel: Center, Top Left, Top, Top Right, Left, Right, Bottom Left, Bottom and Bottom Right.
The Unity manual does not mention this feature, though it does mention an alternate:
Using the Editor
There are also settings for the sprite’s pivot, which Unity uses as the coordinate origin and main “anchor point” of the graphic. You can choose from a number of default rectangle-relative positions (eg, Center, Top Right, etc) or use custom coordinates.
- Unity Manual: Sprite Editor
If I may speculate, I would say this feature has been implemented for the purpose of consistency. The manual does not deliberately mention it, but pressing ctrl to enable pivot snapping is a feature I have seen in other programs, before. While Unity officially directs you to use the control panel, the shortcut key has likely been implemented for users who are already use to holding ctrl. Such implementations are often referred to as external consistency, and helps facilitate human-computer interaction.