Tor was invented with a purpose of giving users with poor, restricted connections to access forbidden stuff untracked. Think journalists under oppressive regimes in the third world, or, of course, undercover agents.

I2P was invented to give people with relatively good, usually excellent, connectivity to access forbidden stuff untracked. Think media "piracy" first and foremost, also dark-grey market stuff, etc. Everything else is better served by either your own VPN, or the public Internet.