These magnets are intended and tested for use with magnetic trays and GW bases as a point of reference. Of course, you can also use them to magnetize turrets, weapons options, and other modeling shenanigans.
These (3/8" x 3/32" N42) are slightly over flush on small bases, which may be desirable. On larger bases they are very nearly flush. When there is a gap, it is highly recommended to use Green Stuff to help hold the magnet in place, as thin super glue (cyanoacrylate) tends to be brittle and the magnets may snap off.