The slots are provided on angle plate for

An angle plate is a work holding device used as a fixture in metalworking. Angle plates are used to hold workpieces square to the table during marking out operations. The slots are provided on angle plate for accommodation bolts.
Slotted holes or "T" bolt slots are machined into the surfaces to enable the secure attachment or clamping of workpieces to the plate, and the plate to the worktable.