Doctor Prescribed me Folic Acid to take with my Topamax, Why?

I have been taking Topamax for about a year now. Today my dose was doubled in MG's per day because I was having seizure/ fainting episodes and headaches again.
The doctor Prescribed me Folic Acid to take daily along with the topamax. He said it helps with taking topamax during and before pregnancy (from the recent findings about birth defects and stuff) And he wanted me to take it for in case I where to become pregnant sometime.
One Problem- I have my tubes tied! I reminded him this, but he still wants me to take it. Im 23 years old, married with 3 young children, and Ive had my tubes tied about two years (since a month after birth of my 2 year old son)
So now, im wondering if this is Really necessary to take- or is it just going to be another pill?
Do I really have to take it even though I cant get pregnant? Is is possible the doctor thinks I could get pregnant? I take enough pills, Im wondering if this is just a silly uunnecessary precaution?
Thanks!!

[FurballsMom]

Well, folic acid is a B vitamin, so I really don't think it will hurt anything.  Many people are low in folic acid.  You can buy it from a health food store or online.  I like the Swanson Vitamin Company for things like that, because of the low cost.  I buy several things at once to make the shipping costs worth it, even though the shipping is low.