Reverse NS allows you to find all domains sharing the same nameservers while reverse MX allows you to find all domains using the same MX records. These come in handy when the domain’s Whois record is private or when the IP address is hidden by using CDNs.
Also, reverse NS is useful when you’re familiar with the nameservers of the top hosting providers in a given country so you can look up all domains hosted there and filter them for your needs: eg niche marketing, B2B research, etc.
