You are the only one at the airport early in the morning. You get a slot delay of 20 mins today. The tower controller is friendly. He lets you out at CTOT-7 (he is allowed only CTOT-5). You have unusual easterly wind and make unother 6 mins because of the departure rwy. Now you fly your 45 mins enroute at max speed max fuel flow. You make together with shortcuts another 4 mins. You approach direct with max speed and make another 5 mins to touchdown (you are again the only one). Now you are proud: you are 2 mins early !!
The sense of the 20 mins slot delay ? Nobody will be able to explain. Perhaps: "...it did serve the needs of the whole aviation community"
Slot delays can be in place for more than only the first or last sector, so maybe in this example the third of fourth of fifth or... (you get the point)-sector was overloaded during the time you were planned there. With you starting 13 minutes late (and not only you but probably a few more who filed that via that part of airspace during that time), you get there after the peak in traffic. Wich can be only a few minutes, but could mean serious overloaded situations if you (and the others who are slotted) would be there as well within those minutes.
And therefore the controller has enough capacity to provide you with dct's etc.
Maybe a day or two at a larger ATC-facility with an FMP-desk would open up your mind a bit.
How far out are you reduced into large US-airfields? How much time do you spend alongside the runway with you engines running at large US Fields? No comparrison to be made here...