Good question! The imperfect (imperfetto) is used to interrupt an action. If you said "sono stato" instead of "ero," what you would be saying is "I arrived at work" and then (while at work), I got wet. With ero, getting wet occurred during the action of going to work. With "sono stato," getting wet occurred after arriving at work.
Why is it ero and not sono stato ?
I'm going to make a post of this! Thanks for the question.
Good question! The imperfect (imperfetto) is used to interrupt an action. If you said "sono stato" instead of "ero," what you would be saying is "I arrived at work" and then (while at work), I got wet. With ero, getting wet occurred during the action of going to work. With "sono stato," getting wet occurred after arriving at work.