alt Hacker NewsUSB-to-Ethernet adapters are life savers when you need to:
(A) replace your WiFi adapter - download drivers from internet
(B) configure a router or other equipment (hard to configure WiFi without WiFi).
(C) stand up your Linux install on your laptop (easiest way to futz around until you get WiFi adapter working - but check chipset on adapter is compatible which the cheapest usually are)
You don't usually care about the performance. Just keep a cheap one in your box of shit - I need mine often enough. If you need high performance, then buy a high performance adapter.
Not saying they're not useful for specific purposes. But anyone buying them hoping to improve performance compared to their WiFi, often comes away very disappointed.
In my case A) and B) are irrelevant because I only really own or deal with laptops now days, and they invariably have built in WiFi, but usually not built-in Ethernet!