A bucket can be used for many things. The tankers could use the bucket to get water from a nearby well or pump to fill their canteens. They could also use the bucket, if there wasn't any running water, to take a crude shower. You could also put water in the bucket and use it for shaving. To supplement their rations, soldiers often ate local food; the bucket could be used to make soup or stew. When performing maintenance on the tank, the bucket could be used to carry tools, or oil or gasoline in lieu of a can.
Here's some photos of American tanks with buckets:
One reason you don't see as many buckets on US tanks is that they were supplied with a folding canvas bucket. Here's a pic of one http://www.germanmilitaria.com/OtherNations/photos/C053364.html
In this pic (last one at the bottom of the page) You can see one folded up just to the left of the bedrolls http://www.ww2f.com/topic/15367-tank-crews-what-equipment-did-they-use/
Commonwealth tankers frequently used cut down rectangular 2 gallon petrol cans as both buckets and as a make-shift stove.
yeah this is still done today. I was an infantryman in a mechanized infantry unit so we rode around in bradleys and tbh we had big gatorade coolers strapped to our bradleys for holding water for the exact reasons that have been outlined: showers, drinking water, brushing teeth, shaving, etc.
