0% inflation is a bad thing because it encourages people to put off purchasing things. That means that people who make things lose work. Fewer people are employed; less stuff is made; the economy as a whole doesn't advance.
Low inflation is good for savers, but they'd rather have people invest money than sit on it. Inflation is a punishment for people who take their money out of the system.
The Fed aims for a gentle inflation to nudge people into buying stuff now rather than putting it off until tomorrow. The 2% figure is, very roughly, the amount of inflation that matches the expected systemic unemployment under Okun's Law. ("Laws" in economics are, of course, much less solid than they are in physics, but it does mean there's a model behind it and not just a guess.)