Because one day someone else may need to read and understand your code?
Of course, that probably depends on the nature of the project. Having all backend code of a web app in one file and all the frontend code in another would be very inconvenient. OTOH, I have no problems with having e.g. multiple React components in one file...
I believe that files offer one useful level of compartmentalizing code that makes other people easier to understand what is going on by just looking at the file structure before opening a single file. The other guy can't grep anything before they have opened at least one file and found some interesting function/variable.