The point of debating is to see how a presidential candidate will respond to challenges from the public, the media, and the seats in congress. If a candidate can't handle the argument on television, how can they handle every question that will come at them in the Oval Office. If a candidate can hold their own in the debate arena, then there is a good chance they will know what they are doing when they get to the white house. The easiest way to judge whether or not they get my vote, is to watch them handle themselves in a debate.
For Hilary Clinton, it is important for her to gain exposure to provide new or re-framed narratives to help her campaign. For Bernie Sanders, exposure helps breed familiarity with some of the more subtle components of his platform. More televised debates on highly-watched nights (i.e., not weekends) will improve visibility and reputation for the candidates and the party.
Media saturation and advertisements often make it difficult to get a real understanding of the candidates. Televised debates provide voters with the opportunity to hear the candidates speak about a variety of issues. The also get to hear the candidates respond to each other. Debates help voters make a more informed opinion.
Americans already get plenty of time to watch the televised debates of the political candidates for the upcoming election. During this time, they can start to learn more about them and decide on which they will support. Airing more debates will only make the country become weary of watching them, and might make them lose interest in the topic.