Focus matters the more you have to switch tasks, the more that focus comes under pressure for software testers or developers.

With a large project working on several features at the same time, a roadmap is very important. This roadmap sets up a high-level schedule that makes it clear to everyone what is currently a priority and what tasks need to be focused on.
Sometimes priorities change, of course, but as much as possible it works best to follow agreements so that functionalities are completed sequentially rather than in parallel. Seen from the point of view of one individual, of course. What should be avoided is that a developer or tester is asked to start working on something else while the previous one is unfinished.
From a business level, a schedule change may seem to have little effect, but for a developer, it means setting aside unfinished code that he has been working on. That he should focus on new documentation now, just as he understood the previous ones. For a tester, it means testing something incomplete after understanding the specifications, to have to test a new functionality, where again different documentation is important.
This doesn’t work. Focus matters, and the more you have to switch tasks, the more that focus will be strained. Confusion ensues and quality declines. Returning to ‘old’ code or to an ‘old’ functionality does not amount to ‘nicking it back’, no, again there is a period of figuring out, where exactly you had left off and what the functionality meant again. Short: double work, double energy, half focus.
A roadmap can change and is usually for a long period of time. That it changes is obviously not unnatural, given that we work in a world in motion. However, it is important for a developer or tester to have a chance to finish his tasks before starting something new. Try to avoid priorities changing so last minute that tasks remain unfinished.