What does the Ignition Housing Assembly have to do with starting the Engine?

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I've been having intermittent starting problems.I changed out ignition switch, coil antenna sensor and no change.had starter checked with meter it wasn't getting voltage. Took starter out and tested at shop it tested good.So We assumed no voltage is getting to starter from relay or sensor or switch.Battery good altI good.Locksmith said key lock and keys are good.Took it to dealer they said ignition housing assembly is bad.My question is what does this housing have to do with the Starting of the engine? Is there a main ground that is attached or power source that cranks engine? ?? I thought the Housing just holds switch key cylinder and sensor.I've never gotten into a wreak or never was ruff with steering wheel.I hope someone can explain this to me clearly.The housing comes with the switch.dealer says I don't need the other parts I can use same ones.Please any help would be appreciated.