If you create your entire map as a single mesh, then your entire map is always drawn if any of the map is visible. If you compose your map of individual meshes, then Godot can cull the meshes that are not visible. You also have easier access to alter parts of your map on the fly.
I’m not a professional, but I have experimented with both approaches and found that composing maps from individual objects is far superior for my games than the all-eggs-in-one basket approach.