Use Flatten Faces to force a series of faces into the same plane.
Flatten Faces can be accessed via the right-click context menu, or via shortcut FL.
Flatten Faces can be useful as a repair tool, when a face that should be planar becomes unexpectedly faceted (with smooth edges between the facets) - either when importing lower-fidelity data like SketchUp models, or after accidentally moving a vertex or edge which causes a face to get faceted/triangulated.
Flatten can also be used as a design tool, to "align" or "extend" shapes by selecting a guide surface first, then selecting subsequent faces to flatten to the plane of the first surface.
Check out the video below to see the power and versatility of Flatten Faces: