Why do teams Draft/Sign QBs with a different skill set to their starter?

This mostly applies to teams with mobile QBs that call a good amount of QB run plays. But why do team draft/sign QBs that have a different skillset than the starter? For Example the ravens that call a good amount of QB run plays and have built their offense around Lamar’s mobility and skill set they then draft Devin Leary who finished his college career with negative rushing yards and they sign Cooper Rush who doesn’t have the mobility to run the ravens playbook. Look at how much better the offense looked with Tyler Huntley running it compared to cooper rush last year. The eagles have done the same thing where while they don’t call a ton of QB Run plays into their offense they still incorporated Jalen Hurts mobility into the offense and then they draft guys like Tanner McKee and Kyle McCord that were basically statues in the pocket coming out of college. We’ve seen it go the other way to where a coach that has an accurate system QB that plays within structure draft a Mobile Inaccurate QB that doesn’t play within structure to be the backup.