That wasn't their main thing. The main one was to buy stocks or companies for less than they were worth, either because the market mispriced them or because company owners wanted to retire and sell to Berkshire. They also were big into insurance float which is getting insurance premium money that doesn't have to be paid out for a while which provides a kind of free leverage. Buffett did a lot of other things too.
I think over the last few decades, he could make deals just because the Buffett name held so much influence. If Buffett was investing, then others could assume that the company won't go bankrupt.
That's certainly part of it, but in the "buying mispriced assets" category, Buffer made some very smart, large bets. During the Great Financial Crisis, BH made a $5 billion investment in Goldman Sachs that eventually made them billions in profit when everyone else was running for the hills.
I actually think this is a deal only Buffett could have made. By bringing on Buffett as an investor, it sent a signal to the market that the bank was solvent and improved confidence in the company. Another investor would not have brought the same calm so would not have had the same return even if they bought the same amount at the same time.
