From the General tabbed page, toggle Snap to grid.
When Snap to grid is on, the secondary cursor snaps to nearby grid points as you move around the sketch. Depending on the grid size, magnification of the viewport, and whether preselection is active, you may also be able to select other points in addition to the grid points and the existing sketch geometry.
When Snap to grid is off, the secondary cursor aligns with the primary cursor as you move around the sketch. You can select anywhere on the sheet.