It’s frustrating I know, especially when you don’t see roadworks/an accident.
But someone explained to me once the idea behind it and it makes it slightly more bearable.
It’s all about cameras feeding to a system where it sees buildups and adjusts the flow of traffic, sometimes many miles before, to try and even the flow and get a greater number of cars through than would happen if they all piled into the build up at the national speed limit.
Although that person had no explanation when I said I’ve seen that many times on the M25 and other motorways at 0200 when going to/from work.