If the snap to grid is activated (menu Grid, options Snap to vertical guides, Snap to horizontal guides, Snap to all guides), and the snap tolerance is set to 0 (menu Options > General Preferences > Guides), wherever you drag, the object edges snap to vertical and horizontal guides (even if the guides are not displayed). If the snap tolerance is different than 0, the object edges will snap to guides only if their distance from guides is smaller than the snap-to zone (in pixels). This means the snapping will occur on distant zooms rather then on closer zooms. For further details, see Snapping objects to guides and grid.