Organic waste going into landfills is a huge problem because it has a climate impact.

If your banana peels decompose in a compost heap, they give off carbon dioxide, which balanced by the carbon dioxide absorbed by the plant to photosynthesize and make the banana in the first place.

If your banana peels decompose in the sealed environment of a landfill, they will decompose via a different microbial process, called anaerobic decomposition (meaning no oxygen). This is a slower process and it it chemically different - it produces methane, which is a much more potent greenhouse gas than carbon dioxide. Landfills have to vent the methane that's produced by this decomposition or else they get a bit splody.