The primary use case for these tools is the acquisition of copyrighted material (films, software, games) without payment. Under laws like the DMCA (USA) or the Copyright, Designs and Patents Act (UK), downloading pirated content is a civil tort. While individual downloaders are rarely prosecuted, the facilitators (the leech site owners) are targets for copyright trolls and legal authorities.
Nitroflare and similar hosts employ sophisticated countermeasures to disrupt leech sites: Nitroflare Premium Leech