You can merge with a blend so that the two surfaces transition into each other more gradually. Or you can make an exact merge so that the new surface keeps all of the original surface data of the two source surfaces.
Surfaces for a merge ideally have the same number of rows or columns so the two surfaces meet. The Exact option is best when the two surfaces share a common boundary curve. When they don't meet, exact inserts a ruled surface between the boundaries being merged.
If you had two neighboring flat surfaces, you could combine them with merge into one flat surface.
The Blend option is best when the surfaces don't meet. A fillet-like blend is created to fill the gap. Merging automatically untrims trimmed surfaces. Merged surfaces may be trimmed again as needed.
The Exact option is best when the two surfaces share a common boundary curve. When they don't meet, exact inserts a ruled surface between the boundaries being merged.