This altcoin tax system is very simple; it's for those legit altcoins that are actually doing it for a good reason and not to scam people with their token.
There are many reasons and purposes for those tasks, which are to reduce the circulating supply, reward holders, and donate to charity. At least that's what most of them do.
They can do it to reduce selling pressure from their users. It was common applied in GameFi projects because with those projects, users, gamers simply have to buy their tokens, buy their NFTs, and play games almost forever.
You see they invest one time, can play to earn many times, forever. It will create very big selling pressure when users already hits their break-even point, they will sell tokens they earned from the games without hesitation because they are already in profit.
However, sale tax is like a scam already. Because firstly projects with their marketing, make their users think buying NFT, playing games will bring a lot of profit. At the end, most of promised profit will be charged through sale tax and the project team take it. What they do with those sale tax income, users can not control.