So they can buy things they don't need but just want. So they can really splurge on their many luxurious vacations for example. People in Europe work harder than Americans. They work fewer hours, but are much more productive than workers in the US.
Nobody should feel "incentivized" to work harder by the threat that they'll become homeless the moment they get sick or need an ambulance. If you feel like you'd never work hard if you didn't have that threat constantly hanging over you maybe you're just unnaturally lazy.
Isn't the answer obvious? So that you can afford cool things that you want. There needs to be a baseline of minimum services everyone gets (health care, education, food). If you want anything beyond that, like a nice house, nice car, nice toys, travel, etc. then that's the motivation to work harder.
Sure you would. Well, I can't assert about you, but most people in that position would.
First, what is usually left out of this kind of discussion is how much work it takes to earn an extra dollar. At minimum wage every extra dollar is a lot of work, so every penny in taxes is a big hit.
When you're making say 1 million a year, every extra dollar takes very little thought or effort, so getting less of each is a non-issue in practice. And when you're making billions, each dollar is asymptotically close to zero effort, so even at a 99% tax rate you'd be getting an extra penny for zero effort, which isn't bad at all when you're getting millions of those pennies.
But, we don't need to keep this theoretical, we can simply look at history.
In 1980 top tax rate was indeed 70%, and that did not stop people from working at making lots of money, buying Ferraris and mansions and yachts. So we know from historical fact that it wouldn't stop the rich from riching. It just makes society a tiny bit more fair.