OK, it's possible that there's a low-profile outlet for releasing news of upcoming discount periods, and I just don't know about it. However, I can list several effects of the sale that occur whether or not shoppers know about it in advance of coming to the store:
- You might choose to buy more of something than otherwise, since it's cheaper right now.
- As a special case, you might have planned to buy zero of something, only to increase that amount when you notice it's on sale.
- You might choose to buy a discounted item in preference to the non-discounted item you were originally planning to buy.
The fact that one $3 thing you bought was unexpectedly on sale but every time you go to the store you spend $200 on other stuff is a thing that the store is aware of.
In Germany, most grocers have little leaflets where the majority of sales are announced a bit ahead of time.
Aldi makes a lot of consumer goods business with that, when they sold their first desktop in the mid-90s, all of them built by Medion in the UK, it was a huge deal with long waiting lines even before the stores opened and them running out of stock in a matter of hours.