The Tor Browser, it’s just a normal Browser with some functionality to improve privacy.
Like many tools, it can also be used for nefarious things, but that’s not its only use.
The use case for TOR is illegal activity. Some illegal activity is not immoral, like organising a protest against a dictatorship. But Tor is not a useful tool for simply browsing websites. The inconvenience isn’t worth it when a regular browser fulfills your needs better.
It’s like money laundering. It could be done recreationally, but that’s not the normal use case.
Tor isn’t explicitly developed to promote illegal activity. I’ts just another browser with some more layers, just like an Onion.
It’s more than just privacy. It allows you to visit .onion sites, which will not load in a traditional browser. As a harmless example, this is Duck Duck Go: https://duckduckgogg42xjoc72x3sjasowoarfbgcmvfimaftt6twagswzczad.onion/. Trying to click that in a normal browser doesn’t work because they don’t support the onion network. But using the Tor browser unlocks that as well as all sorts of nefarious sites that you can’t access through a “normal browser”
