Lots - probably most - money is taxed twice when in a single person's possession. Once when it's earned, once when it's spent.

When looking at different taxes, you have to consider what the government is doing. Of course they are raising money to operate, but there's more than that - they are usually trying to encourage or discourage certain behaviors. An estate tax discourages families from piling up absurd amounts of money - keeping money "working" in the economy is a good thing. But at the same time, since it only applies after a certain amount, it still allows people to be concerned about their family after they're gone - especially when it comes to things like keeping the family home, or funding children/grandchildren's educations.

It happens with a lot of things. Tobacco is a big one. In my state, a lot of tobacco tax goes to schools. That's great - except the two things don't have a lot to do with each other. The government wants to fund schools, and make it shitty for people who choose to smoke. Another one is tax benefits for married couples. The US used to be almost entirely empty, and given the morals of the time, the government wanted to encourage people to get married... and have kids, and settle the country.