From the beginning SW had part file,assembly file and drawing file.Later (SW2004) we get possibility to have several bodies inside one file. But after this I think SW went wrong way:they made weldments which contains a lot of bugs up to now (SW2018):

I found out that PTC Creo use usual assembly for weldments.In this case it is not necessary to make cut-list, their cut-list properties,derived parts with transfer of cut-list properties to file properties and so on.

Assembly can provide all functionality weldment have.

So why SW made one more essence (weldment) which born a lot of problems and bugs?