Owner Buying Business A's Items/Supplies Through Business B

I apologize if this isn't the correct subreddit, but for years I've wondered about the purchasing scenario that occurred at a company I worked at and am hoping to get some insight.

So years ago I worked at a security company (Company A). I was an employee of A, wore a shirt with A's logo, and drove a truck with A's logo. The owner of A also owned Company B and for all service parts, tools, general hardware, even trucks Company A would always buy from Company B.

For example if I was low on truck stock for batteries or needed 2 replacement cameras or motion detectors I would call the office and say I need (2) ABC123 and head to the parts house. I'd get an email in a few minutes with a PO for the parts under Company B, and I'd walk up to the counter and say "here to pick up (2) ABD123's for Company B". From my understanding Company B would but them at say $100 each and sell to A for $125 each and then A would sell them to the customer for $145 each. My understanding was that all tools and testers and most parts and hardware we had were bought from company B.

I do know that on big RFP installs company A was buying direct from distribution due to deal registration savings. As far as I know B didn't sell to anyone but A and B's employee list was just the owner of A&B, his wife, and their daughter.

What would the purpose of this setup have been? It was by no means a sketchy company that paid bad or treated employees bad. It was a great place to work which just makes the situation even more confusing in my mind because there was nothing else weird or red flag-ish going on.