Check out the proficiency table at this site:

https://roll20.net/compendium/dnd5e/Character%20Advancement#content

Also, read this explanation: https://www.fandomspot.com/dnd-5e-expertise/

As you can see, proficiency bonuses are what makes classes more distinct. Without expertise, rogues and such are not much different from anyone else. With expertise, they are clearly set apart.