Limited supply prevents hyperinflation, which is a major problem with fiat. Humans cannot resist the temptation of having too much of the good stuff, thus making it less valuable in the end. Bitcoin cannot be issued or found more than expected and technologically possible. This takes the power from people and institutions, at least in a way not letting them make the currency less valuable. A lot of economists think it's a bad thing, though. They argue that if money gets more valuable, people are not encouraged to spend it ("I won't buy it today, because it will cost me less tomorrow" thinking). However, this assumption presumes perfectly rational agents, and people are not like that. In practice, there just haven't been to many cases of deflation to see whether it's a good model. There have been lots of problems with inflation, though, but economists are blinded by their theories, so they don't seem to care.