I can follow the chain of causality to the real-world value creation from them. I can't do the same with the ponzicoin of the week, where people are only making money because other people are losing it. With no causal chain that leads to anything getting produced.
For a particular example of an interest rate swap, anyone whose business incomes are closely correlated to short-term loans, and whose expenses are closely correlated to long-term loans (or vice versa) would care deeply about them, because they allow them to hedge themselves against unpredictable changes in interest rates.
