[mbivert]

Agreed.

For example, once you have a good grasp of the codebase, and an overview of the future requirements, you can perform low-risks, local refactorings, so as to ease implementing both current and later features/bugfixes/etc.

The requirements aren't systematic though. Meaning, as a dev, you're not always, at least explicitly, allowed to get a bird-eye view, nor to act on it.