There's plenty of reason. For example, it's often very important whether deals are made between "arms-length" parties, especially where a valuation is involved. If a corporation is allowing me to buy their property for $1, it matters whether that is the fair value, or if the corporation just secretly belongs to my brother and we're avoiding taxes on asset transfer.
The fair value can be assessed independently of that, though -- and frequently, it is anomalies in values relative to fair value that lead to investigations that reveal the non-arms-length nature of a deal.
It's often very difficult to assess fair value when it's not determined by an arms-length transaction. (That's one of the challenges faced by command economies). And even when it can be done, it takes a lot of work to do. Even the price of oil, or shares in a company, can fluctuate on a daily and regional basis. In an individual transaction it's even more difficult -- is this a loss-leader? Or a free sample? -- and when it's a unique, non-fungible good, still more difficult. The price of used assets sold at auction are all over the map. And for some assets there is very little active price discovery in the marketplace. That cryptocurrency that went up in price 1000x shortly after it changed hands, was that just a lucky call? Is that patent really worth $500k? That commercial property that's been without a tenant for nearly a decade, is it really worth an imputed rent of $40k/month when nobody has been willing to rent it at that price? ... But if these two companies signed a contract that values it at that figure as collateral for a loan, then maybe that's the market price? As they say, location location location, right? Etc.
The arms-length principle is not literally a requirement to be unrelated. It is a requirement to act that way. If one brother sells another brother a moving truck for 40000 USD, it is an arms-length transaction. If he sells it for 1 USD, that is not. It's a non-arms-length transaction because of the value, not because they are brothers!
In some of the cases you mention, it's difficult to assess fair value even when it's determined by an arms-length transaction, so the sudden disclosure of a relationship would not make it at all straightforward to take anyone to court over it.
The commercial property without a tenant for nearly a decade isn't worth 40k USD / month. When you say "...these two companies signed a contract that values it at that figure as collateral for a loan...", you are describing fraud. That could arise not because of any plan of fraud prior to the loan agreement but because of one formed during it!
