How exactly can someone have millions of dollars and be of zero use to society?

In order to get that money they either had to have earned it by selling goods and/or services, or received it from gifts or inheretanc from someone who did.

If they put their money in the bank, it isn't really useless because the money will be invested by the bank, so the economy will still grow.

If they use it to buy a lot of useless things, the money changes hands, so whoever sold him the junk will use it to buy what they want and need.

If all the money is put in a briefcase and buried, then forgotten, that is effectively useless to society, assuming that the money is never found and spent.

Money should be a unit of social credit, I don't think the problem is some people make more than others for seemingly trivial things, I think the problem is the divide between preceived benefit to society and actual benefit to society.

Of course some humans are worth more than others - explain why an alcoholic junkie stealing for another fix is worth as much to society as a normal working man

100% agree. We should all earn what we are worth and no human is worth more than any other.