Many years ago, the directors of a company called Aspro asked themselves the same question. They had overwhelming market dominance, something like 90% of all UK aspirin sales, so why bother with advertising? For a couple of months sales held up and it looked like they had done the right thing, but then, suddenly, sales collapsed. They never did recover.
Because, generally, their products are 'generic' and sold by multiple other vendors at cheaper prices (Lidl etc.), and advertising is proven to sway the minds of shoppers (and convince them to pay more)
The problem 'generic' sellers is that unless they keep advertising a 'new boy' can eat their customer base before they know it ... eg Woolworth V's the 99p Stores
Because marketing is the lifeblood of every company. Some bother some don't. Those are well established because their marketing strategy is spot on.
One is to compete with the other massive supermarkets and secondly that is how they stay so well established. Also they can tell you about all the good offers they got going on.
Because the all have competitors and if they don't keep their profile high
their competitors would literally steal their place in the market they are in
eg retail, soft drinks, grocery etc
It keeps their name out there, and helps them make money.
