[j]

If you already know the answer then I’m not sure why you are asking the question. So, with the caveat that it depends on what type of fitting you are talking about and I assume you are talking normal Shedule pipework here then it is down to the schedule of the pipe not being of uniform thickness on for example, a tee or WN flange. So, if you cut off the prepared beveled end of a tee to put in your tie-in then it is impossible to prepare an new bevel to match the ID of the inserted pipe or fitting exactly, thus having a ‘lip’ inside the pipe.

Of course if you’re talking Biobore tube for example then this wouldn’t apply.

Well, Did I get it right? Do I get a gold star if I do?