There is no denying that account structure is super important. There are many accounts that I have worked on where an account restructure has dramatically increased performance. How? Good account structure improves an account in 2 ways.
First, it clearly defines to the advertising platform (Adwords, Bing, etc.) where you want your ads to show. This means that your ads should be serving how you expect them to and your keywords are not competing with each other in the auction. This ultimately should result in showing the right ads to the right people.
Second, it makes it easier to manage. You know where things are and the organization allows you to know what your account optimizations will actually do.