Leaders; I get it, but I don’t get it.

[fish4942]

So I understand why leaders are used on the end of different lines, but I don’t get why. That probably doesn’t make one bit of since to anyone, but here’s what I mean…
I see anglers using 50lb braid line for flipping and pitching heavy cover with a 20lb fluoro leader. I understand the leader is used so the fish aren’t “spooked”. But doesn’t the 20lb fluoro undermine the purpose of the 50lb braid? To me it would seem like the braid has been compromised because of not only the 20lb fluoro, but the knot used to join the lines now becomes the weak link. Why not just use the 20lb fluoro altogether since it’s the direct connection to the fish? I know there are mega knots that people tie with excellent strength and great non-slip factors, but the leader is subject to the same hazards as the braid when pitched in the heavy cover / weeds/ brush, etc. Or why not just use the braid only? I just don’t get it. What am I missing? Do most of you use a leader with your braid?