Why would a company (A) create another company (B) within itself?

Both companies are literally in the same building.

Company A is manufacturing plastic products and company B is all about company B's IT (sys admin, dev, etc.).

Probably economics or plans to expand their customer base or focus area

E.g. to get around certain regulations that apply to companies of a certain size or larger. Or to get around labour agreements. Or to fuck around with where exactly the companies are registered and use tax incentives. Or to let one of the companies deliberately go bankrupt (probably the IT one) in order to outsource work to Cheapistan, but avoid paying the fired workers their termination pay.

Tax evasion.

And everything that @Fast-nop said.

To fuck with laws and regulations.

To scoop up more money into someone's pocket

When the tax laws gets complex companies get complex.

Several reasons, most common is, I think, when a company gets bigger and bigger and starts to have different products and serviced that no longer make sense ti have inna single roof, also accounting is easier when things make sense.

Another reason us better control of vision, mission, strategy and employees.

Different countries allow fir different strategies, I some you can just define sub groups inside, in others you have to officially split the company.

Also, if one company is delivering services or products to the other, you can deduct costs, do tax avoidance and even money laundry.

So yeah, several reasons.

Some kind of scheme for money funneling.