If my last child was delivered by c-section. Why is that important for them to know if I plan vaginal birth?

Pre-planning. The routine delivery will anticipate a need to shift to a C-section if progress stops or the baby shows signs of distress. It takes some time to switch but they do it.With a prior C-section, they often staff the delivery unit with extra personnel & a surgical suite already prepared (nearby) because there is an increased risk that a repeat may be needed more acutely.
Slight increased ris. There are more and more deliveries being done despite previous caesarian. You really cannot hide this due to the incision from the caesarian. Delivery can be accomplished vaginally without too much risk but your OB will be aware of this and be a little more attentive to the risk of uterine rupture which is a serious problem.