It's used for range extension: the repeater takes the signals from your primary wireless network and repeats them on its own network. In addition, the router can serve as a bridge.

Range extenders cannot be used to increase Internet throughput from public hotspots (They can be used to increase available signal to Wi-Fi devices, such as with the PR2000 Trek travel router and range extender, but this will not increase speeds.) You cannot connect an extender to another extender's wireless network. The differences between a WiFi extender, WiFi booster and Range extenders rebroadcast on a different wireless channel from the one used by your wireless router. The challenge with range-extenders is location. Incorrect placement means a range extender may not obtain sufficient WiFi signal from your router to simultaneously rebroadcast, stream and engage in online gaming or watching a video. The Best Wireless Range Extenders for 2020 | PCMag Jun 12, 2020

Jun 12, 2020

Extend Wi-Fi Network Using Old Router as Repeater

How to Use TP Link Router as a Range Extender or Wi-Fi

While some wireless routers (and access points) can also function as a repeater, you need to take into account for every repeater you add to the "chain" you effectivly halve the speed/bandwidth. A better way to acheive greater wireless coverage would be to run cable, and have an access point instead of a repeater.