There are three primary reasons why people look for older battery stock:
For vintage laptops (10+ years old), original manufacturers often no longer produce replacements. Finding NOS is sometimes the only way to get a battery with the correct proprietary shape and fit. 2. The Risks of Chemical Aging
Used or older stock batteries are significantly cheaper than genuine new manufacturer replacements.
Hobbyists often buy old laptop battery packs to harvest their internal 18650 lithium-ion cells for custom storage solutions, power banks, or electric vehicle projects.
3 surprising things you can repurpose laptop batteries for - XDA Developers