It's an easily recognizable icon that indicates which room to use and doesn't rely on which language you speak. For English speakers M and F might be just as easy, but which would you use of حمام للرجال and حمام للنساء?
Funny story about this is I was in an Irish-themed bar here in the UK and the toilets didn't have symbols but Irish words.
I was in a rush and saw "Mna" and "Fir". Mna sounded like a drunken "man" so I went with it. Once I'd finished my business, however, I noticed there were no urinals and it was the women's toilets. I had to laugh.
I was in a rush and saw "Mna" and "Fir". Mna sounded like a drunken "man" so I went with it. Once I'd finished my business, however, I noticed there were no urinals and it was the women's toilets. I had to laugh.