My personal experience
After taking over the campaigns, I worked on restructuring and optimizing the account, and within that period, the number of conversions increased by around 3x. The client started getting much better conversion volume and overall results.
After about two months, the client decided to manage the Google Ads account himself instead of continuing with me. However, once he started managing the campaigns on his own, the performance gradually declined.
Around five months later, he came back to me and said that he was no longer getting the conversions he used to and asked me to check the account. When I audited it, I found several mistakes and issues that were affecting the campaign performance.
After fixing those issues and optimizing the campaigns again, the conversions started improving once more.
So, this was one of my experiences that really showed me how important proper account management, continuous optimization, and having someone experienced monitoring the campaigns can be.