Really I'm talking about any arbitrary directory within a file system, including spaces where files are shared between users, so not necessarily within a project directory. But I think your point gets to the heart of it - usually we want everything to be contained relative to the project directory so we are not referring to things like
"C:\Users\brianstamper\...\my_data.csv" because if we share that code then it isn't going to work for someone else. But sometimes we need to refer to (or create) files that are stored in fixed locations in some common file system, and to keep that information from being buried somewhere deep in the code I like to somehow declare the location near the beginning of my script, either in a function like above or even just as a character string.
Another way I have done this is through symlinks, like
if (!file.exists('data')) file.symlink('/some/common/data/location/', 'data')
But to people not familiar with symlinks that may be a little cryptic.
Obviously this isn't a major issue and there are some workarounds, I just came to the thought because I was writing something where I was doing a lot of file creating/reading in one of these directories and was pining for tab completions.