It not only has to do with blood pressure, but the fact that MORE blood is going through all the blood vessels of the body, including the eyes IOW veins are bigger etc.
A lot of women's vision correction changes slightly during pregnancy due to the amount of fluid within the eye, and/or the amount of blood in the vessels of the eye, hormone changes, and thickening of the cornea.
They are most often nothing to worry about (preeclampsia happens in 3% of all pregnancies), and they are quite common.
Its always good to mention the spots, as others have said when accompanied by high blood pressure and other PE signs the doc can keeping an eye on things a lot sooner.
When I had PE it was dull headaches along with the slow moving big round circles in my eyes that indicated my complications.
I have little sparklies (flies)

and wavy lines this pregnancy and do not have PE.
