bridge library confusion

I am very confused that I can just upload sketches without Bridge library, and it works well just as UNO does but wirelessly, so what is bridge library exactly for?
I know it's for the communication between two processors, where people can use URL command or shell command line to control the pins on the board. As we know, the wifi module is connected on Linux processor, so only Linux processor receives the sketch first then transfer to 32U4, from my view, it seems like we have to use Bridge library in every sketch.
Can anyone help me figure it out? Thanks in advance.

You can load sketches wirelessly without using the Bridge library. If all you want is an Uno that can be programmed remotely, you don't need the Bridge library in your sketch. You will end up with a system that does pretty much everything an Uno can do, including not being able to communicate with the network. If that's all you want, then you don't need the Bridge library. You will still have the Linux system, and that will be able to use the network, but it will have no communications with the sketch, and the sketch will not be able to use the Linux resources.

The bridge gives your sketch many additional possibilities:

  • The ability to have a serial I/O accessed over the network without a USB cable (the Console class.)
  • The ability to accept simple web requests, usually used for a REST API (YunServer/YunClient classes.)
  • The ability to make outgoing web requests (HttpClient class.)
  • The ability to accept incoming TCP stream connections (YunServer/YunClient classes.)
  • The ability to make outgoing TCP stream connections (YunClient class.)
  • The ability to set/read key/value pairs for AJAX JSON access (Bridge class.)
  • The ability to accept queued up messages from web requests (Mailbox class.)
  • The ability to run Linux commands and scripts with two-way communications (Process class.)
  • The ability to read/write SD card files (FileIO class.)
  • And probably others that I'm not thinking of right now...

To be fair, all of these can be done without the Bridge library - if you want to do a lot of extra work. While not always the most efficient way of doing things, the Bridge library makes these tasks pretty simple.