First, you understand that for feature creation (extrusions, cuts,
revolves, lofts,..), using sketches (closed or open profiles), have to
be solid (not hidden lines) and construction lines (hidden) can not
define a profile. (and yes, a centerline or construction line also
does define a rotation center)
Think of construction lines (dashed) as reference entities within your
normal sketch (solid),.. lines, arcs, curves,.. which serve to
reference construction intent and drive your sketch entities (solid).
In your sketch, you can always toggle from hidden to solid and back
The most common or simplest example of a construction line is used
with polygon sketches, such as a closed rectangle sketch, where you
apply a diagonal construction line to quickly define the center of the
rectangle, (via the mid point of the construction line) or using a
construction circle to help define the center of a hexagon.
Use construction lines to help construct of your open/close profile
such as,, you may want to only create a layout sketch, 2D or 3D, which
could define your profile intent or open/closed profile boundaries...
so, you may have construction entities within your layout sketch to
help with defining your open/closed boundaries.