A couple of reasons:
The approval is likely contingent on you buying a lot with utilities, sewer, etc. The situation will change if you want to buy a parcel that does not have water, electric or sewer to it.
A good Realtor will not require a pre-approval letter before working with you, but they may call your lender and ask about the parameters of the loan program. You actually would prefer to have a pre-approval letter geared to the exact property you are making an offer on, since any open letter now would have the highest dollar amount that you could qualify for. Wait to get the letter until you know the price of the property you are bidding on, otherwise the seller will know that you can qualify for more.
Real estate broker, former morrtgage broker