meaning of come in

[Learning_English]

come in (BE INVOLVED) phrasal verb INFORMAL
to become involved in a situation, story or plan:

e.g. We need expert advice, and that's where you come in.
I can't understand neither the meaning not the example.

Could you explain me the parts in bold, it will help me to understand the phrasal verb "come in" altogether.
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Please let me know if I have made any mistakes in any of my sentences.

[Mister Micawber]

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'Where you come in' = the point/time/situation at/in which you contribute/participate.
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Native English teacher at Mister Micawber's

[Learning_English]

[Mister Micawber]

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