give one reason why a lap flange and stub end would be used instead of a weld neck flange?

Answer 1

One common reason is because of cost. If you need to use a very expensive or hard to get metal for the pipe then the backing flange can be carbon steel like astm a105. 

Answer 2

They are used in piping systems with critical wall thickness requirements.

Source(s): https://www.unifiedalloys.com/blog/lap-joint-flang...

Answer 3

Because it's cheaper ?