For me, it's a friendly competition.
After my engine gets its initial rating, I intend to improve it and move up the rating list. That obviously entails running matches against other engines. For me, the fun is in seeing my own engine improve and play better chess due to the things I program. Thus, just copying a piece of code (such as the new NNUE code in Stockfish) to add 100-200 ELO to the engine is no fun. The engine is stronger, but it becomes stronger using code and techniques I do not (yet) understand, thus the strength increase would have no value to me.
Writing the chess engine is basically a competition against myself, by each day learning a bit I didn't know yesterday, and the increased engine rating is the proof that I've learned, understood and implemented the technique correctly.
And, because it's a friendly competition, I have no problem with explaining a technique to someone else if they can be helped with that, if it makes their engine stronger. I will learn techniques from them as well, or other people.
