Why do you measure the winding resistance with a multimeter?

Actually, measuring winding resistance with a multimeter is an iffy situation at best. This is because the multimeter will not tell you if one of the turns is shorted, causing Q to drastically suffer, it will really only tell you if the winding is open, or if it is shorted to something it should not be shorted to, such as the frame.

This is especially true for larger windings, in larger Transformers and motors, because their DC resistance is so close to zero that you might not be able to tell if they are shorted or normal.

Using a multimeter is a good start, to make sure that basics are covered, and it might tell you if the winding is dead shorted, but you need to know for sure what the expected DC resistance is, and you need to know that you multimeter is very good in the low Ohms scale.