As a law-abiding citizen, what use is TOR and dark search engines?
For the moment, it remains pretty niche, I won't say otherwise.
Tor is necessary for accessing some legal websites that might be of interest to you, like KF when it's down because of trannies.
Tor is useful for circumventing censorship in your country to access websites that are blocked, like some Chinese or Russian websites that they don't want you to access.
Tor is useful for privacy. Even if you don't do anything illegal, you might not want the government to be able to know that you have this or that opinion, or do this or that on the Internet. You don't need to have done anything illegal for them to take interest in you.
And even without breaking any laws, you might be interested to know that the best, cheapest, safest way to get drugs is on darknet markets. Plenty of drugs are not illegal and Tor is the best way to get them. When I lived in Europe that's what most smart people did actually.