The main point of this composition is to have the two winston jump on site and put the shield, two shield on the holywood first point (one on the left of the payload and one on the right) will cover the whole road to the site. You have two tracer because they can do well against almost any champ alone and against champ like thorbjorg they also can do well if they coordinate. So they can follow the winstons (they jump on site fast so better have some blinks) and clean the site while the shields are up.
So you probably understood, that here, the speed is one of the major key (with the shields), one lucio will make you able to rush site and jump on it with the speed boost, with that speed + tracer blink + winston jump, you can go on site in few second even if ennemies have advanced position. Since you go really fast, they don't have time to prepare and organize the defense (The composition 2 tracer 2 winston 2 support is particular and defense must adapt to it, and your objective as offensive team is to not let them time to adapt.)
So to sum up:
2 Winstons to cover the way to site with shields during their jump.
2 Tracers to follow Winstons and counter play a lot of ennemy with good coordination.
1 Lucio to speed boost and don't let the time to organise defenses.
The last support is more likely to heal, either lucio to heal the whole team and if first lucio die you still have a speed boost available, either mercy so you have a faster healing but lucio needs to survive.
I'd add that if you have two support here, it's because you want to abuse speed even once on site so the first lucio will keep the speed boost up.
The comments on the second post remembered me of something I forgot to precise, since I can't comment, I'll edit my post:
Two Lucios is really great when well coordinated, but this require such a coordination I didn't saw in not premade team even near the top of the ladder. Because you need to swap to use the Volume Boost, one lucio isn't only healing while the other is only speeding, you have to swap to use 2 identical boost one after the other. If you don't have the coordination, one Lucio probably won't be as efficient as a Mercy. I recommand to use 1 Lucio and 1 Mercy except if you know you are well coordinated and with a premade.