The primary direct use of a surface plate is to be a reference datum for measuring and layup. If you want to measure how high something is in thousandths of an inch, you need a very flat surface to measure from.
More importantly, if not something most people actually do themselves anymore, all the precision equipment (mills, lathes, etc) needs precision surfaces. The same basic techniques that you use to make a surface plate are used to make the ways so that the bed or carriage moves exactly parallel or normal to the axis of rotation of the quill or head. These are Vs or dovetails instead of simple planes, of course, but the same ideas hold. At the core, they are made by reference against a surface plate.
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More importantly, if not something most people actually do themselves anymore, all the precision equipment (mills, lathes, etc) needs precision surfaces. The same basic techniques that you use to make a surface plate are used to make the ways so that the bed or carriage moves exactly parallel or normal to the axis of rotation of the quill or head. These are Vs or dovetails instead of simple planes, of course, but the same ideas hold. At the core, they are made by reference against a surface plate.