Sometimes songs are performed in different keys too.
Sometimes bands are not tuned to A-440 (think de-tune Eb or D) but the band learning/playing the song aren't going to bother with all that involves.
Also, sometimes people have different interpretation of what is being played when left to only learning a song by their ear. This is like saying, here's what I/we play so Rob can match & compliment what is already established & going on.
It also opens up the opportunity to discuss potential changes or improvements & saves time during practice. Ya know like, 'what the fvk happened during that chorus part, something sounded weird & not like the record I listened to'.
Last, it is a courtesy & time-saver. Not everybody is as advanced & accomplished or quick to learn by ear as Robert & even when they are, $#it can still go sideways during a song for all the many reasons listed above.
The Relics was a bunch of well accomplished learn by ear players that still ran into song format (getting turn arounds & verse/chorus structure correct) issues that could send one person off in their own direction while the rest of the band went that-a-way!
OK Ray, I've said too much, again.