WiFi extenders are incredibly versatile network products. 
Below are some ways in which they are used:

• Range Extender: Extends the range of an existing Wi-Fi network or hotspot
• Access Point / Wired Network Converter:  Provides wireless network connectivity to wired Ethernet devices
• Wireless Bridge:  Wirelessly connects 2 or more network segments
• Minimize Mobile Broadband Data Consumption:  In areas where public Wi-Fi is available (and/or Mobile Broadband is not) simply connect the Range Extender to the Mobile Broadband Router (i.e. Cradlepoint Routers), and switch your internet source from Mobile Broadband to Public Wi-Fi without changing settings on multiple connected laptops, smartphones, etc.