Experience is the best teacher

Proverbial Meaning: The best way to learn a job is to do a job. 

Literal Origins: Sometimes you attain qualifications or train for a task, but it's experience that will make you more proficient at the task. 

Common Usage: This proverb isn't heard spoken exactly, though most English speakers will know what it means, and the sentiment is there in every "Established since 1976" etc by a company's or skilled worker's name.