I would say it would be in case you haemmorrage after labour. I am prone to anaemia and haemmorage during pregnancy and birth and have to have a cannula in when I am in labour, JIC. I also have terrible veins which are difficult to find at the best of times, and when I haemmorage they are nearly impossible to get to. If you do have one put in, make sure it's in a good place, definately NOT the crook of your elbow. The best place for me is kind of in the back/side of your wrist below your thumb. That way you will still have most range of movement and can hold things without too much trouble.
As far as I have been told (as recently as my last birth a few weeks ago), iron levels of around 110 are pretty normal. Anything below 80 and you could be a candidate for a blood transfusion. So 90 is pretty low considering you haven't lost any blood or anything. I am surprised the nurse didn't advise you to take iron tabs. If I were you I would definately be asking a professional (doctor or midwife) about it as soon as you can. I tend to get anaemic when pg too, and started taking iron through this pg when my iron levels dropped to 102.
Good luck hun, and take care.