plate numbers

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hello,
         why is there so much attention paid to plate #s, and why do the printing co.s even bother with them?
 
Also, I've noticed with the 37 series 50s that fpn and bpn #1 for both is quite common, and the 54 series 50s , #3 both sides is common.
 
 thanks

Among other things (which I will leave for other members to mention), plate numbers help establish a chronology. For example, with the 1954 $1 notes, CBN started printing with plate #1 on the A/A prefix. As time went on, the prefixes incremented, as did the plate numbers. I have a note from the X/O prefix with plate numbers 115/115. If you found a note without any serial numbers, printed by CBN, and with Beattie/Rasminsky signatures, you could look at the plate numbers for information on when it might have been printed. A lower number would indicate that it may have had a denominational letter of L. A higher number might indicate O.