Question

explain why the accepted value for the beaker volume is 50 mL but 50.0 mL for the graduated cylinder?

Answer 1

It has to do with the fact that the graduated cylinder is more accurate than a beaker. For example, with a beaker you may be able to observe 59mL of water, but in a graduated cylinder, you would be able to observe 59.8mL, which is a more accurate measurement than what the beaker is able to show you.
Basically, the more zeros a measuring instrument has after the decimal point, the more accurate that instrument is going to be.

Answer 2

Thank you @valia