It means a lot more on a PC that has room to waste. It works on Arduino but you run out of RAM before necessary.
I have a parse&lex word-match function that easily keeps up with on-the-fly high speed serial. It does not require a buffer or any string.h functions. When I feed it text from RAM, it averages 9us per char matched while walking through a word list in PROGMEM, the list can be 1000's of words long (2000 words with average taking 8 chars fills a bit more than half the flash on an Uno, a Mega can support loads more list) and not bog this code down.
There's no way I would think to write that kind of code if I thought in terms of String or string.h.
For most things text, I don't need or want an approach where string.h or String functions need to be used.
Buffer then process requires more RAM and that's just the start when using those approach/technique-limiting functions.