One more reason is that if you have JMP 2000 and your program is moved in memory 1000 (octal) bytes forward, you automatically have JMP 3000, as it remembers the offset, and not the exact address value. You can use an exact address with JMP this way: JMP @#3000. It will not change when the program moves.
So, relative addressing allows to run the same program,...
The wiki page PDP-11 architecture has a cryptic paragraph with no
references or examples:
Over the life of the PDP-11, subtle differences arose in the
implementation of instructions and combinations of addressing modes,
From Bitsavers the PDP-11 Systems Handbook 1987, Appendix A-PDP-ll Family Differences Table states:
The table ...