[OE-core] why does BBFILE_COLLECTIONS get assigned with "+="?

[Robert P. J. Day]

  not sure why i never noticed this before, but why do layers *append*
to BBFILE_COLLECTIONS rather than just assigning the layer name with a
simple assignment? is this just to allow alternate names for the same
layer? can't believe this is the first time i ever noticed this. is
there an actual example where this is being used?

rday

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Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
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