[long_time_gone]

==So if company A buys company B, which has assets of $50 for $100, then they'll add $50 in goodwill to account for the difference.==

Goodwill is an asset and we frequently see it monetized. It isn't just a made-up number to make things balance, it is a stand-in for particularly "hard-to-value" assets like perception. Ford famously licensed their logo and built a $1 billion business [0]. Prior to the licensing deal, that value would have only been captured as Goodwill on Ford's balance sheet. It is the value of the blue shield that they have built over decades of company performance and advertising.

==But it does mean that you can't use goodwill to accurately estimate the effects of brand advertising as there could reasons other than brand marketing for a company being traded above its assets' fair market value at the time of the sale.==

Goodwill is a combination of many things, one of the largest pieces being brand value. Publicly traded companies generate a Goodwill number each time they release a financial statement.

[0] https://www.forbes.com/sites/dalebuss/2012/05/24/ford-has-bu...)