There are only about 1,000 reasons. The most obvious is that a survey marks the corners of your land. From the corners you can determine where fence lines should be, where a pool can be built, and all of the other dozens of decisions a homeowner might want to make.
If you are buying in a developed neighborhood, and a home is already constructed on the lot you buy, the survey will show you setback lines, and from those you can determine whether there are violations of the minimum setbacks provided by zoning ordinances or restrictive covenants. Setback violations can cost you money if you resell the house and if a possibility exists that they must be corrected at some point in the future, even, if the possibility of correction is remote.
Each of the above could be the subject of articles, or chapters in books, and will receive no more treatment here, except to say that they could be important.
Recently I reviewed a transaction in which a buyer took title to a vacant piece of land. The buyer received no survey. After closing the buyer learned that the property was 100% wetlands. That is to say, even though he could walk on the property when he bought it, and even though there was no water on the property at the time he bought it, it is defined by Department of Environmental Protection definitions as "wetlands." If you bought that property for investment purposes, or to build a house on, your chances of doing that are dismal. It also means that even if you are successful in building or in holding the property for investment, the expense to the owner of mitigating or correcting the wetlands condition of the property may be greater than the value of the land.
By law, a survey must delineate wetlands. The surveyor must also show all other encroachments on your property. That means if he sees a road going across your property, you will know it from the survey. If he sees someone else's retaining wall or embankment projecting onto your property, you will know it from the survey. If he sees your own improvements encroaching onto an adjacent lot, you will know that as well. That could be important because your own encroachments onto someone else's lot maybe required to be removed. You could encounter thousands of dollars in expense in removing problems of that sort that could have been avoided had you done a survey.
Any time you purchase water front property, the surveyor will show you the "mean high water line," which is typically your property boundary on the water side. He will also indicate on the survey improvements built into the water. Those improvements might be a large part of the reason you buy waterfront property. Whether or not the improvements are within the confines of your riparian rights is something only the surveyor, or a lawyer with experience in riparian rights and the projection of property lines into the water, could answer.
The most important reason you need a survey is that any title insurance policy you receive will have a survey exception in it unless you have a, current survey. That means that whatever may be shown by a proper survey, if you had one, will not be insured by the title policy. Give yourself plenty of time to obtain a good survey. Don't call 3 days before a closing and expect a surveyor to get out to the property, run the property lines, and prepare a neatly drawn complete survey before the closing date.