Having to drink multiple glasses of water and then not relieve your bursting bladder is not one of life's great experiences. So we don't ask you to go through that without good reason. It comes down to physics. The type of sound waves used in medical diagnostic ultrasound are pretty fussy about where they'll go. They simply will not pass through bone, nor will they cope with any gassy area. That's why ultrasound will never replace chest x-rays (air in the lungs surrounded by ribs) and are not very useful at seeing broken bones.
If your scan is to look at a body area where there is often gas, e.g. in the pelvis behind the small bowel, then a full bladder helps to push the bowel loops out of the picture and gives a clear view of why lies below it e.g. an early pregnancy, or a troublesome uterus or ovaries. A trans vaginal (TV) scan (using a slim probe a bit like a tampon introducer placed inside the vagina) can get around the gas works, giving the delicate sound waves a short trip to the target of the scan. The draw back of the TV scan is the short range it has; it may not be able to find the ovaries or other items of interest.