More often than not, code is read than that written. For a project, you might just spend a short while developing the code, but you might spend several hours reading it repeatedly.
Following PEP-8 guidelines will make your code easier to read and understand, saving you time trying to figure out why it was created in the first place as well as allowing you to focus more on the next challenge in your project.
Anyone who collaborates with others or works on similar projects needs to use PEP-8; without it, even assisting a friend on their projects can be quite challenging.
Nothing is more frustrating than working on a task and not knowing why a certain piece of code is there or what it does. By requiring everyone to follow the same guidelines and practises, PEP-8 helps to avoid this issue and produces code that is not just legible and uniform but also totally intelligible.