1. Wide range: The subway is one of the important parts of urban transportation, and a large number of people take the subway every day, so advertisements can gain a wider audience in the subway.

2. Good visual effect: billboards in subway cars are usually large-area posters and billboards with bright colors and clear patterns, which are easy to attract people's attention.

3. Long time: The time in the subway is longer, and people spend more time in the subway car. Compared with other places, it is easier to stay in advertisements, thus increasing the exposure of advertisements.

4. Low cost: Compared with other media, the cost of subway advertising is relatively low, so for some small businesses and advertisers, subway advertising has become a more economical advertising method.

5. Strong regionality: subway advertisements can target different target groups according to different subway lines, so they have strong regionality and pertinence.

To sum up, there are so many advertisements in the subway based on the consideration of market demand and advertising effect. By placing advertisements on the subway, we can gain a wider and more diversified audience, increase advertising exposure, and enhance brand awareness and sales.