I think he meant "how" [to get 'weld.rb'], not "why" [he divided the edge before welding its parts]...
To use the EEby... tools [and others] any profiles/rails/etc need to be 'curves' - that is any collection of edges that have been made into a 'polyline' [in CAD-terms] - or a 'curve' in SUp-speak - SUp's arcs etc are naturally 'curves' - Bezier curves etc made with Fredo's tools are also this type of 'curve' - BUT any edges joined end-to-end [with no branching] can be made into such a 'curve' - even if there is no apparent 'curviness' in it at all 😕
The tool to make these edges into a 'curve' is weld.rb. However, you can only 'weld' two or more edges into a curve, if you have just the one edge then it can't become a curve that way - therefore select the lone edge, right-click context-menu > divide it [i.e. 2 or more - if it's to match a 'paired' curve as a profile/rail/etc then it's recommended that you divide it to have a matching number of segments], it will remains highlighted [but it's now in bits], immediately weld those selected bits into a curve...
You are now ready for he next step...
🤓

@gaieus said:

If you just type the number (as Remus suggests), it will set the degree. You can also give the value in focal length if you type mm(millimetre) after the number.

If you set the focal length in mm you will afterwards still set the focal length in mm. Doesn't seem to revert back. No in my experience anyway...

What you "marked" as an error is in fact not an error. Projected texture should behave like that (that's why it's called "projected" as if it were coming from a computer projector; it will be "distorted" on sides near parallel to the "projector rays").

Basically "wrapping" is what you need instead (Remus solution is something like that) although it is rather hard to do on such heavily triangulated surfaces you have (see especially images 3 and 4).

No, but SWEEP 2 RAILS plugin will be very usefull for me!

Pilou, you don't have to double click then deselect the face. Just click the face and then use Follow Me. The profile will follow the perimeter of the face. 😉

See here for a tut I made a couple years ago:
http://www.sketchucation.com/forums/scf/viewtopic.php?p=4286#p4286

Or here on the new SketchUcation Site:
http://www.sketchucation.com/?p=1265