Why In a linked repeater network, two or more repeaters are tied together by RF links, microwave, or the internet so that whatever one repeater hears is retransmitted by every repeater in the system. This lets a low-power handheld cover a whole state or region, because your local machine passes the audio to all the others. Statewide emergency nets and wide-area club systems commonly work this way.
Watch out A single repeater with multiple receivers is a voting or satellite-receiver system, which extends coverage on the input side only, not a linked network. APRS is a packet position and message reporting system, not a voice linking method.
Linked = one in, all out. Talk into one repeater, everyone on the network hears you.