Why do the Chinese RMBs use characters like 壹圆 instead of 一元？
一 is a simple character, right? Say I borrow Mr W’s pen and give him an IOU which says I owe him 一元。 I leave the IOU with Mr W, of course. No sooner have I left than Mr W takes that pen and with one stroke, my debt increases tenfold: 十元。
The formal numbers, 大写， are used because they are not easy to alter once they are written down. The formal numbers are words that are pronounced the same (or almost the same, in the case of 6 and 8) and are put to use as numbers.
Here, get out your pen and copy 一二三四五六七八九十。 I’m sure that if you get imaginative, within seconds you can alter those. Now try it with the formal numbers: 壹贰参肆伍陆柒捌玖拾。 Good luck.
Do you think that if I wrote 壹圆， Mr W could change that to 拾圆？ Not likely!
元 and 圆 are pronounced the same. As far as I know, the original word was 圆 round, referring to round coins. But that takes more time to write than 元， so that is used for informal use, simply because it is quicker to write, and if you have a sign or something, easier to space and to see from a distance.