Not all 2D shapes are created equal in Blender: create a circle via Add – Mesh – Circle and it will be fundamentally different to creating one with Add – Curve – Circle. Even though, they look exactly the same in the viewport.
The trouble begins when you want to do path related things on 2D shapes, such as turning a path into a pipe. That’s not possible with 3D meshes (and the first circle we’ve created is seen as a mesh, even though it is only two dimensional).
To alleviate this problem, Blender can convert a mesh into a path, and a path into a mesh. This can be done with Object – Convert To. The handy keyboard shortcut is ALT + C.
All paths can be converted into meshes, but not all meshes can be turned into successful paths (for example, when they’re not two dimensional).