An RTL_TCP server first of all is a taking a USB Software Defined Radio and setting it up as a server for receiving radio signals within the frequency parameters of the SDR device which can them be connected to from anywhere. For example an RTL-SDR Version 3 operates from about 500 kHz (with direct sampling enabled) to about 1.7 GHz.
So if you set up the server you can be anywhere provided your server allows incoming connections to the internet or you can access your network via VPN, as I do, and connect back to it and hear all the local radio stations or radio signals that interest you.
What good is that? Maybe there is a radio program you like but the station doesn’t stream. Maybe you want to hear your kid playing his high school football game broadcast on local radio. Maybe you set a microphone and transmitter up in your home as a security device (such as a baby monitor). Maybe you want to listen to the local weather broadcast or maybe you are a scanner junkie and like hearing your local police scanner. Whatever. There are lots of reasons.