Default IRB interface in SRX Role?

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The SRX comes by default with two interfaces (fxp0 and Irb0) as shown below:

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fxp0 {
unit 0 {
family inet {
address 192.168.1.1/24;
}
}
}
irb {
unit 0 {
family inet {
address 192.168.2.1/24;
}
}
}

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The fxp0 is the OOB management port but what is the use of the irb0. I really don't get it.