Fat! If you put it on the rack, all the fat and juices will drip into the bottom of the tray, leaving you a less fatty leg of lamb, but you would need to baste IMO more often to stop it from drying out. The fat will keep the meat moist and juicy. If you put it direct into the pan with the potatoes (part way through the cooking process), the potatoes and any other veges you add will absorb the fat in the skin making for nice crunchy bits. The meat should be moist and not need basting through the cooking process (although do check).
I do roast lamb (either as legs or steaks/chops) semi-regularly and always put meat direct on the tray. I add some veges at the start of the process, depending on the veges and don't add oil to the pan.
My favourite baste for my lamb is balsamic, honey, garlic with some herbs and seasoning.
Also consider that a leg of lamb direct on the tray, can be very tasty without any messing around, maybe just some pepper. On the rack, additional seasoning may be needed as the fat adds flavour to the meat. Alternatively, you would have a really lovely start for a gravy!!
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