The mesh on the solid seemed to work without share topology, but in general, it is good to use share topology when you have separate surfaces that are touching at edges. However, many CAD systems have a Sew operation that connects separate faces into one large surface body.

If Share Topology is successful, all the parts will be connected by sharing nodes at common edges and there will be no need to use contact. You can color the edges according to the Connection and the color code will tell you if the edges are properly shared. Red means a free edge, black is normal, purple is like a Tee intersection where three elements touch an edge and Yellow is like a Cross intersection where four elements touch a common edge.

Here is the SolidWorks help on Midsurface.

Once you bring surface geometry into Mechanical, the mesher will automatically create quad elements in the mesh and apply SHELL181 or 182 elements by default.

If you have a cantilever and one end of the beam is fixed, then you can find the first six natural frequencies and mode shapes by requesting six modes.  If you have no fixed support, you have to request 12 modes to see the first six natural frequencies as there are six "zero frequency" modes before any non-zero bending modes show up.