A structural ridgeline creates a minimum amount of sag (the U shape) of a hammock. That lets you lay on the diagonal easier, because the material is looser.
If you have a hammock without a ridgeline, the tighter it is pulled the more 'inline' you have to lay in the hammock due to the tension, and if it's tight enough the sides will squeeze your shoulders.
Read the ridgeline section of this page
, for visuals.