One may want a braid leader or tougher or heavier braid topshot to help against rocks or abrasion. One may have a thinner or cheaper bottom backing due to budget or line capacity, depends on the fishing or situation. I think a mono leader before the terminal gear is to prevent loosing line and polluting with braid (which is a nightmare for sealife and swimmers etc. and has caused drownings of divers), one would make this mono lighter than your mainline so when you bust off you just leave the terminal rig behind and don't break your mainline off randomly somewhere where it has a nick. Also breaking off heavy mainline and an even heavier braid leader and very heavy mono/wire trace which is jammed up, is actually a huge mission and can cause injury without some tool to wrap your line on to break off(I've quite badly injured myself)..
Ja but for edible rock and surf..I am now over the braid for that application. The mono is much more suited to the environment and fishing..and fighting of the fish among in the rock and surf. The reels are more robust and last longer. I am grateful to those that dragged my stubborn ass into the multiplier world.