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 π
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...
π€
A