The most dominant type of compression used in genesis titles, LZSS compression, compresses 50% better with packed pixel formats.

The advantage of a packed pixel format is that it lends itself to compression better, which makes sense on a console like the genesis which had a fast processor and could handle graphics decompression during runtime. You want 32 colors in packed pixel? You HAVE to store it as 8 bits, with 3 bits wasted per pixel. To put it another way - you want 32 colors? In Planar format, that's 5 planes, with each pixel being 5 bits. Packed pixel formats must conform to the size of a byte. In other words, planar graphics expand by a power of 2. What this means is if you want an arbitrary color depth, say 5-bit color, then 1 pixel is only represented by 5 bits, there are no wasted bits. Planar graphics are stored 8 pixels at a time, across n number of planes, where n is the bit depth. A tile itself is, for the contexts of these systems, 64 pixels, drawn in an 8x8 grid. When we are talking about pixel formats like planar (NES, SNES, SMS, Game Gear) or Packed Pixel (Genesis), we're talking about the way pixel data is arranged.