Asking my own zettelkasten this question: (responses in no particular order as individual affordances are sure to vary in usefulness by user; some framed as problems while others are framed as affordances, the difference should hopefully be clear to most):

• information overload
• mitigating time loss and context collapse in regularly interrupted work
  • Acts as a ratchet and pawl for thinking work
• tool for thought, and a particularly inexpensive one
• a catalytic surface for thought - see creativity
• removal of cognitive bias by allowing direct juxtaposition of ideas
• "a plan for life and not just a book"
• creativity acceleration
• artificial memory storage/improved memory
  • spaced repetition tool
• improved search by indexing ideas over time
• Clarity/specificity: it's a reminder to be specific about what you're thinking
• a system for marshalling resources (collecting)
• new context creation through context shifting and/or erasure
• "slow burn" productivity
• focusing attention
• fun
• serendipity generation
• attempting to look cool by doing what the cool kids are doing (this usually results in failure modes however)
  • related: fear of missing out (FOMO)