The 'chip' as you call it is flash EPROM. It is not replaceable and holds all the programming for the car.
When you modify the car near a point when the ECU can not compensate, that is when you need a 'tune'. The ECU can adjust fuel up to ~25%, but don't count on it always doing this. And, just because it can add more fuel does not mean the quantity of fuel it's adding is correct. Nor can you say the ignition timing is correct.
When you get a tune, things like the fueling requirements, timing adjustments and air flow corrections can be made. These are done to correct any error the ECU may be compensating for and ensures it is doing so correctly.
A throttle body swap does not allow more total air into the engine. But it does slightly change the rate at which air enters, which is throttle response.
Throttle response is how quickly throttle input translates into moving forward. A larger throttle plate allows more air into the engine when going from closed throttle to open throttle. The reason for this is surface area and differential pressure.
The atmospheric pressure is what forces air into the engine. The only reason this can happen is the pressure in the intake manifold is lower than that before the throttle body. It's the same thing that creates wind/weather. The larger the differential pressure the faster air can move. The larger the opening the greater the quantity of air.
I know that'll be confusing. Google the terms I've underlined and you should get a better handle on how it works.
