I saw a cryengine video in which an edge loop is dragged to adjust it just like the ones in paint. I am not sure if that feature is available in Blender. If yes, would greatly help in modelling objects like bridges, arches etc. The link to the video: https://m.youtube.com/watch?v=0X123xPlyWM the curve is used in 00:18 of the video.
Unfortunately, blender doesn't have a magic button that can do this so you have to do it manually.
- Select the edge loop and press Shift+D and Esc to put it back
- Press P, seperate by selection
- Select the new mesh and in object mode, press Alt+C and choose Curve from Mesh/Text