Set while the concrete is still soft, not drilled in after
The whole reason a J bolt looks the way it does is the timing. It goes into the wall or the slab while the concrete is still wet and workable, pushed down to the depth the hardware schedule calls for and left standing at the height and spacing the framing plan needs. Once the concrete sets around it, the hook is buried in solid material on every side, and that is what gives it its holding power.
A bolt drilled into concrete after it has already cured works a different way entirely. It goes into a hole bored with a masonry bit, gets anchored with an expanding sleeve or an adhesive, and holds by friction or bond against the sides of that hole rather than by a hook cast into the mass of the concrete. Both do the same basic job, holding a sill plate or a piece of hardware down to a slab or a wall, but a J bolt only works if it goes in before the pour, and a drilled anchor only works after.
Why the hook does the holding, not the threads
The threaded end of a J bolt is for the nut and the washer, nothing more. What keeps the bolt from pulling straight back out of the concrete under load is the hook buried below it, turned so the concrete has to shear or crush before the bolt lets go. That is why depth and embedment matter more than how tight the nut goes on top. A short hook set shallow gives a wall framer far less to work with than a properly embedded one, even though both bolts look identical sticking up out of the concrete.
One shape among several in the anchor bolt family
J bolt is a shape, not a category by itself. Anchor bolt is the broader word for anything embedded or drilled into concrete to hold a structure down to it, and that family includes straight rods with a plate welded on the bottom, L shaped bolts, and post installed anchors set after the fact. A J bolt is simply the hooked version most commonly used for a standard sill plate, chosen because the hook does its job without needing a plate welded on underneath it.
Set to the framer's layout, not wherever is easy to reach
The bolts do not go in at our discretion. Spacing, the distance off each corner, and where the hold-down hardware clusters near openings all come off the framing plan and the hold-down schedule your builder or framer hands us. We set that layout before the pour, because a bolt that ends up in the wrong spot on a wall that has already cured is a problem for a drill and an epoxy anchor to solve, not a hammer and a chisel.