A land surveyor's main job is to mark off boundaries of a property, and this type of service can be needed if you have a dispute with your neighbour about where a fence or other such structure might be erected. However, there are many other jobs done by a land surveyor and other reasons to hire such a professional, even for private residential property. Note a few of those reasons here so you can determine if and when you need such a professional and how they can help you.
You may know where your actual boundary falls between your property and your neighbour's property, but you may be accused of encroachment. This refers to building something too close to that boundary. Many areas and especially residential areas have encroachment laws in place, so that you don't block a neighbour's view or run the risk of damaging their property with your own structure, oversized tree, and the like. A land surveyor will be familiar with encroachment laws in your area and can note if your proposed structure would actually encroach on your neighbour's property or violate those local codes and can assist with reworking your plans if needed.
If you have a very large plot of land, especially if it's not yet cleared and developed, you may want a map created for the area. This map can assist with knowing the location of certain trees, water features, natural landmarks, and the like. The map might be used to then assist with parcelling the land so that you can sell off parts of it, or with planning the layout of a farm, location of a new outbuilding such as a barn or shed, and other such features.
Water boundaries can be needed for any property that is adjacent to a water feature, even a small one. You may want to fill in a pond in order to develop your land and may find that the pond is within a neighbour's boundary. You may also find out that you have rights to a certain body of water, when you were told that the water feature was fully contained on a neighbour's property. A land surveyor doesn't just survey land but will also help determine water boundaries for any body of water, so you know if this water feature encroaches on a neighbour's property or if you have rights to access the water yourself.