I don't think I've intentionally ever used the interlock stitch, although I can certainly see its usefulness. I'd like someone to show me step by step, how it can be done by hand, since my very basic sewing machine will only do straight and zig-zag stitches. I've done hand variations on the overlock stitch, but have never tried the interlock.
Ideally, I'd like to have a sewing machine that would do an interlock stitch, but I don't have the kind of cash that kind of machine would require for purchase. Perhaps, one of these days, I’ll be able to afford one.