“Deep web” and “dark web” are NOT interchangeable terms. There is actually a difference between the two.

The deep web is any part of the internet that is not indexed by search engines such as such as websites that gate their content behind paywalls, password-protected websites, pages behind logins, etc. As an example, when you login to check your bank account, your using the deep web.

The Dark web is .onion links. Not all .onion sites host illegal content, some host perfectly legal, safe, and useful content, and you should only really visit the legal sites, which are not dangerous at all.

The reason people use Tor is because the Tor Browser is superior to every other browser when it comes to anonymity. It bounces your traffic through a network of relays, so no relay has enough information to reconstruct what you are doing on the internet. Normal browsers usually just send your traffic "directly", meaning your ISP can see, to some extent, what you are doing online, and the hoster of the website you are visiting can see who you are (more or less).