Separated waste is worth more than co-mingled waste. Every year, 30% of recyclable materials are sent to landfills because they have been contaminated. For example, paper sorted from collection points is often commingled with other materials such as cans or glass. If it was separated, this could all be recycled.
Paper reprocessors need paper that has been specifically separated and are prepared to pay to get this. The same goes for companies who reprocess glass and cans. If you sort your waste, you can reduce contamination and increase value.