Hi again.
Well you got it partly right. The speed profile isn't created by the ISP, it's created by the exchange equipment and determined by the quality of your line/connection and what speed it can support. I'll try and explain in a simplistic way.
It's not really a case of wasting bandwidth on the ISP side, it's if too much data is sent to the exchange and your line can't cope, then bits will be dropped at random, thus creating errors and the need to retransmit data to correct it. The effect of which is a slower data rate.
Where there is Traffic management, traffic with a lower priority will have bits dropped, slowing down that data rate, so that there is the capacity for the high priority traffic. This is why Plusnet have a profile at their end, matching the one created by the exchange, so Streaming, Gaming and Browsing which have the highest priorities get to you with minimum errors, and hence you get a better experience.
The speed your line can cope with depends on how far you are from the exchange, and the quality of the cable. This is measured in the first instance by the attenuation (the resistance of the line in a simpler way), this increases with length. The longer the line, the slower the speed.
Noise on the line is referring to the general background interference that can disrupt the signals on the line. The best analogy is two people talking in a room, on opposite sides of the room. When the room is empty (no noise) it's easy to hear the other person. If other people come into the room and talk to each other (noise), it then gets more difficult for you to hear the person you are talking to (on the other side of the room) and them you. There is a limit to how loud you can talk (the power of the signal) so if there is too much noise you mishear what is being said so you talk more slowly and repeat what is said.
HTH. If you'd like to understand it in more technical detail, drop me a PM and I'll explain a bit more and then give you a few links of things to read.
