I'm glad someone finally explained this! It makes some sense, but I'm curious, was the combining of the bits of the plane to get the index to look-up done in hardware? Because it sounds like storing 3 5-bit pixels in the expected way in 2 bytes (with a bit left over) wouldn't be any harder in software than combining 5 bits from different planes. But if the hardware handled assembling the various bits, then I guess it would probably be faster. Am I understanding that correctly?
On the Amiga it was definitely hardware, the Denise chip, which could do all sorts of weird and (debateably) wonderful things in addition to the straightforward use case you describe. Hardware sprites being at the useful end of the scale, HAM being at the not-so-much one.
Even if it was done in software, it was still easier to write against planes because you had to run on hardware with wildly different hardware capabilities, so drawing routines were far easier to parameterize over planes than over various bit packing mechanisms.
