The Surface USB-C to Ethernet and USB 3.0 Adapter lets you connect to a network using a wired Ethernet connection. This adapter works with devices that have a USB-C port that supports data. With this adapter, you can also connect to older USB devices too.
What is the use for USB to RJ45 ?\?
The purpose of those devices is to provide an ethernet jack for a computer or laptop. Newer laptops have a habit of not including a network port and there are some computers (usually older ones) that do not come with a network port either.