To be explicit about why, for others, this means your shell will search for executables in a 'bin' sub directory of whatever directory you happen to be in BEFORE it searches your normal path.
This allows for common commands like 'ls' to be executed from ./bin, if they're present, instead of /bin (from your system).
Once you've done this you've opened yourself up to an attack where you download a zip from the internet, extract it, cd into the directory and type 'ls' and you may have potentially executed something from that zip which you didn't intend to do.
tldr - relative paths in your $PATH is a bad idea.
Yeah, I wondered if I should add a caveat in there about this. I forewent it because I thought it would confuse people more than anything, and because I've never found this particular concern all that worrisome for the average developer. But maybe it's worth mentioning, in a footnote if nothing else.
... and let them take care of setting the local paths, and you never have to generate binstubs manually either. (YMMV, of course, but this works pretty well)
