I assume from the numbers people are talking about haemaglobin levels for measuring anaemia rather than iron. You can be iron deficient without being anaemic and that can cause tiredness and other problems like restless leg syndrome.
My Hg was down to 8.5 before surgery, and that's when they gave me two units of blood ( I felt so much better after that ). 3 or 4 months later I had to see the doctor again as I'd become unbelievably tired; I think it was down to B12 but he said my iron (ferritin) was low even though I wasn't anaemic - a healthy Hg of 15, and had me take tablets for another month. Unfortunately for the follow up they didn't bother checking iron or B12, just the normal full blood count, which seems to be the default test doctors reach for.
So really it's just like B12: they shouldn't be letting it get low enough to start causing problems like anaemia, but they do.