It's just a convention: "w/" is a standard abbreviation for "with," so it reads nicely. (The * ending is also a convention.) Arc does have some syntax, though (e.g. f1:f2 or x.y); I posted a list of all of the syntax at http://arclanguage.org/item?id=4012 , if you're curious.
I haven't done any I/O yet, but it looks like you want w/infile and w/outfile.
