Constituents should hold their political representatives accountable because that is the only way that a democracy can work properly.
In a democracy, the people are supposed to be sovereign. Of course, in a large democracy the size of the United States it is not possible for the people to run the government directly. Therefore, they have to elect representatives to do their will. But the important phrase here is “to do their will.” The political representatives have to do what the people want (at least most of the time) in order for a country to be democratic. If the representatives get into the government and then simply do whatever they want with no consequences, there is no democracy because the people’s will is not being heard.
If the people hold their representatives accountable, it ensures that the people will actually be sovereign. Representatives who do not do what the people want will be removed from office and representatives who will listen to the voice of the people will take their place. Thus, by holding representatives accountable, constituents ensure that the people are sovereign, which is a major requirement in order for democracy to exist.