[Evolution] Logic behind a line through deleted messages and copying them to Trash?

[Evolution List <carlos gonzalez shaw ca>]

I just noticed today that the emails I delete and send to Trash show up
with lines through them, like one would have through a deleted word in a
word processing document for instance, as well as in the Trash folder. 

Can someone help me understand the logic or usefulness of both putting a
line through the deleted message in the folder in which it is found AND
putting a copy in the Trash folder?  Why not one or the other but not
both?  

I've never encountered this "feature" before and am curious if there is
a practical use for it.  

I am aware that I can hide deleted messages and thereby "remove" the
messages having a line through them from view.  But it would seem that
there would be some usefulness to viewing them otherwise there would be
no option to seeing them.  

Thanks. 

Carlos