Does it only use in wireless network?

No, you can add a bridge to make layer 2 connections between other interfaces. For example you can bridge ethernet ports together to work like a switch.

It does use CPU for this function though so it is better to use the interface's 'slave' function to create layer 2 connections if available on the RouterBoard (supported with the switch chip). If you slave ports together, they are using the switch hardware instead of CPU cycles.

Tom

one more usage is creating 'loopback' IP interfaces =)

Is it equal to "ebtables" in Linux?

bridge filter and bridge nat in RouterOS is like ebtables in Linux
bridge in RouterOS is like bridge in Linux =)