Drink a ton of water before 4d ultrasound?

Ly

I have a paid 4d ultrasound on Tuesday & they told me to drink a ton of water before it. Im going to be 33 weeks then. Is there any reason for this? They told me to drink 64 oz for 3 days which is the normal amount but I don’t usually drink that much usually so it’s kind of hard! They also told me to have an uncomfortably full bladder