In a deflationary economy, holding money becomes more valuable over time, so why would anyone spend it or invest it if you're guaranteed 5% - 20% more per year by leaving it in the bank?

One way to think of the economy is by money flowing between people and industries. The more the money flows the healthier the economy, as it becomes easier to create companies, get a job, etc.

When the money is stagnant, it becomes harder to convince people to spend it.