Unfortunately, a lot of lenders use estimates for common costs like property taxes, homeowners insurance, and tax prorations. These estimates can be off by thousands and can cause two potential problem scenarios.
The first being if the lender estimates the costs too high and you don't have enough verified funds to close. This will get you a denial letter and prevent you from buying the home you want for no good reason other then the lender being lazy and using estimates to do bad math.
The second scenario is if the lender estimates too low causing you to bring more money to close than you anticipated. This scenario is more common and can really make things hard on a first time buyer.
The worst part about this second scenario is that you don't really find out if your lender used estimates until maybe a week before closing. Imagine finding out four days prior to buying your home that you need an additional $1,000 or $2,000 dollars to close. This causes a lot of uneccesary stress about where you're going to get the extra money needed to buy your home.