Using error bars on graphs is a good way to help students see that, within the inherent uncertainty of the measurements due to the instruments used for measurement, the data points do, in fact, lie along the line that represents the linear relationship. The bottom line, though, is that the trends that lead to more exact relationships are not going to be obvious if all we do is connect the dots on graphs of collected data. You might end up with a drawing that looks vaguely like a bird or a snake, but not something of scientific value.