It's a huge group of people (over the days) with a common purpose, to queue and walk past the catafalque in Westminster Hall. We don't know their individual motives, but they all went there to do that and it isn't Stalin's Russia, no one has made them.
As many have pointed out, the demographic is very diverse. People in wheelchairs, blind people, young and old soldiers with their medals, middle aged women and their Mums, men in morning suits, Beckham, babies in slings, people crossing themselves in both the Catholic and the Orthodox way ( to respect a Protestant monarch) a Native American in a warbonnet, young black men in gym gear, women in saris and hijab. I have not been watching for more than a few minutes at a time but I saw all these (not Becks, admittedly, missed him 
)
Whatever their purpose, they are doing the same thing at the same time and it is fascinating and for some of us, very moving.