While the meaning of "source" may be intuitively obvious, it's still relatively unfamiliar as "src" is far more prevalent than "source" when referring to source

While the meaning of "source" may be intuitively obvious, it's still relatively unfamiliar as "src" is far more prevalent than "source" when referring to source files. While "id est" may be equivalent to "i.e.", you'd still naturally pause when reading text using the former instead of the latter, because the latter is far more prevalent in usage than the former.