Advertising on the sides of buses, inside tube trains and in rail and bus stations helps to keep the cost of fares down. In television and the press, it is a strict rule in Britain that advertising must be distinct from the other material surrounding it. This is why on television, there is always a short break between the programmes and the commercials. In newspapers and magazines, advertisements that might be confused with news stories or feature articles have to be labelled as advertisements. So you can always tell whether it's an advertiser who is talking to you, and you can make up your own mind whether or not to take any notice.
Advertising costs money.
The media sell their space or time to advertisers. Producing the advertisements themselves is the work of highly-skilled people who have to be paid for their time. All this time and effort would be wasted unless the advertiser's message reaches the people it is intended for. For example, if you wanted to advertise a new range of women's fashion shoes, it would be no use taking space in a magazine read mainly by men. If you were selling double-glazing, you would not want to advertise in a magazine for teenagers.