A stand-alone dimensional tolerance (such ±0.1) does not control the shape criteria. For instance, think a section of a cylinder diameter of Ø10±0.1. If we want to measure the diameter, we always keep callipers perpendicular to the cylinder axis in order to measure correctly.
Now, think again a bended cylinder. This time measuring procedure will be same again: we have to keep callipers perpendicular to local section axis of the cylinder in order to measure correctly. With A stand-alone dimensional tolerance, we can only control the sectional deviations like that. If we want to control deviation of a cylinder axis from linearity, we have to use on more tolerances beside the dimensional tolerances. These are GD&Ts.