Jigs can be a great tool for drift fishing. One of the biggest advantages is they fish straight up and down under your float.
Picture this, your fishing a roe bag or piece of wool on a leader, now obviously these items are not heavy, so what tends to happen is they are more inclined to be influenced by the water current and thus drift laterally or side to side around your weight. Thus in order to precisely fish individual current seams and such, you need to be very good at controlling your drifts. Even then, I find I fish specific seams with a few drifts rather than just one or two and move on.
With the jig, you know exactly where your hook is and whether or not you are drifting it through the water you want within a few inches, rather than within the length of your leader as with lighter terminal items. I highly recommend the jig as a search tool for steelhead and salmon alike.
To answer you question ck, I like to fish the jig on a leader below a weight, but as steely points out, they work either way.
I have fished jigs periodically over the years, and with success, but in the early season when fish are less plentiful, I feel the jig is a great way to search out sparse fish in precise lies.
Good luck out there to anyone fishing jigs, make sure they are of solid construction if you are going to fish them. Bent rods jigs fit the bill quite nicely.
Don't forget the other terminal items also have their time and place,
Rib