I will try to answer this, however i don't know the correct english terminology.
The main reason for a tapered leader is the control you get over your fly. With a thick line, you flick (or roll, here comes the terminology issue) the tip of your rod, and you can see the "wave" travel forward through your line, until it "whips" at the end.
If you try this with a very fine line, you won't see the wave travel at all, you will likely just pull your line sideways.
Try create a whipping sound with a shoelace, then try to create a whipping sound with thin yarn. The energy can't travel through almost weightless material.
Now back to the leaders. You want a (relatively) heavy line as far as possible towards the fly, to have maximum control. A quick circular flick in the tip of your rod will let your fly "jump" 10cm left or right if you are skilled enough, which can be the final necessary step to get a fish to bite.
However, you also want to have a very fine, almost invisible line near your hook, to not scare fish away.
If you attach a short, fine line directly to your main line, and whip your rod, the fly on the thin leader might jump uncontrollably. If you attack a long fine leader to your main line and whip, the thin line will just not transfer the energy, and your fly will do nothing (maybe be pulled through the water a little bit).
You also don't want to have 5 lines of different thickness knotted together.
The solution: a tapered leader, which will transfer the energy to your fly smoothly, with no big steps between two lines of different thickness. The energy can travel undisturbed and the line close to the hook is thin and invisible.
If you don't want or have to control the fly in a distance, you probably could just use a longer thin leader directly attached to your main line, for example when you use large streamers for northern pike (don't forget your steel leader ;)).
But by controlling the smaller dry fly you make it jump small distances, just like a fly would after falling in the water. After all, these movements are what makes shy trouts take the bait after all, not just a small something that is motionless floating by