Some people have services only payable through Bitcoin. Mind you, they are legitimate services. I've never bothered with the dark web nor will I ever. I just find it a convenient and in-a-way "safe" thing to use than services like PayPal.
Right. paypal got large transaction fees unlike bitcoin where you just pay a little satoshis to pay online services. Those satoshis converted are just like cents to dollar. Bitcoin would be the right choice.