A one party government is usually a one opinion government. There is no such thing as liberal and conservative in a one party government although smaller, practically invisible parties may arise on the periphery. A one party government would be essentially a political and philosophical dictatorship. There would be no give and take of opinions and therefore no choices for the citizens of the country to make. When the Senate and the House are of one party and the President of another Party, this provides a good check on what either side might do. It's even more effective if each house of the government is of a different party and the President is of either party. Then you have checks and balances also. Sometimes it causes a stalemate but that can be broken, in a democracy, by public opinion.
Most notably obvious in the American political system a divided government provides a tried-and-true system for checks and balances. In the American system there are three separate branches which each review what the others do and have the power to stop a measure which might otherwise be passed with no question by a one-party rule. These checks and balances provide political and social insurance for the constituents of the government.
Although having a divided government provides different views from both sides it does not work well today and it is not what the Framers intended. The Founding Fathers never intended for there to be political parties and by creating these we cause ourselves to be in gridlock, with out getting anything done. Also with increased party polarization and each party drifting more to opposite ends it is hard to get anything done and actually help the United States. Instead we sit still and get nothing done. It is better to get things done with checks and balances rather than have divided government and get nothing done.