I've been using Signal PINs for a long time to lock the app to my SIM card and unfortunately they are a real pain. This sounds exactly the same.
First off the app is incessant about asking you to enter your PIN to prove you know it; this prompt is supposed to get less frequent and I suppose it does but is still way too frequent. Some of us are competent at storing secrets in a password manager and this is like a punishment for us because it can easily take several minutes to go retrieve the strong password, copy it, paste it in.
It actually reduced the security for me; I started out with maybe a 16 or 20 digit PIN and cut it to a smaller number of digits that I could memorize. (I still haven't memorized them!)
There were also issues with Signal's implementation of the feature. I chose quite a long PIN (>20 digits) at first for security only to find out later from them that it was above the silently imposed limit. Later when I went to verify the PIN it would not work because whatever silent truncation was done when establishing the PIN was not re-performed on verification so it did not recognize the PIN.