Whats Up with all the Shouting?

Asalam Alaikum,
When I first came to Pakistan, all during the day I heard men walking up and down the street shouting all sorts of things. I thought it was pretty strange at first, and every time I heard someone I would ask my husband or my SIL what the man was saying. It turned out that they are like door-to-door salesman. They walk up and down the streets shouting out what ever food/service they are selling, then you come to your door and buy what you need.
There are fruit sellers, "Kayla, Seb wala!"(Banana and apple seller)-
Vegetable sellers-"Sabzi Wala!"

"Timatar aur Khira, pandra rupe kilo!"(Tomato and cuccumber, 15 rupees a kilo)
The Milk Man, or Dudh wala, he doesn't shout, but goes only to those houses on his route and rings the bell. You give him your milk boiling pot, or a jug and he gives you a kilo(or however much you normally get) and he brings your bill at the end of the month.
There are Snack Wale-They also don't shout but usually have a tape playing, kind of like an Ice cream truck. This is a popcorn wala-

And this is a Gol Guppa(pani puri) wala-

I have learned how to make these at home, but rarely do, because it is a lot of work to make enough puri for the whole family!

This is the "Teen Dubba Wala!" -

He is like the neighborhood recycling man, he walks through the streets and you give him old newspapers, plastic bottles, old dishes, tin cans(basically anything reusable/recyclable) and he gives you a couple of Rupees by the weight of whatever you give him.