3. to influence undecided voter
The key reason why politicians spend lots of dollars to advertise during elections is to influence voters that are undecided. In every election, some voters already have a preferred candidate for the position available to run for. Usually, this can be based on the fact that they belong to the same party. Sometimes, it can be based on the fact that they have the same ideology. In the case of undecided voters, it is important to advertise aggressively as this might be the only way you can get them to know you.