Might I recommend you read "One Up On Wall Street" by Peter Lynch. He is (was as far as working is concerned) a " buy what you know guy" and ran Fidelity Magellan when it was ringing up returns like the Ark funds in the 80's and early 90's.
I’m guessing you know the following story, but it’s instructive to your point and, I think, helps clarify Lynch’s point.
In 1963, a warehouse operator in Bayonne, NJ owned a large number of soybean oil tanks. The soybean oil, stored in large tanks, would serve as collateral for, in essence, loans issued to the warehouse operator.
Every so often, a representative of the lenders would check how much oil was in the tanks. They’d climb up and insert a measuring stick at the top of the tank to see if the tank was full, and they always were. The problem with their measurements, though, was that their stick only went A yard or so into the tank. And the operator knew this. So, Mr. DeAngelis would fill his tanks with seawater and place a relatively small amount of oil on top (which, of course, floated), thus deceiving the inspectors.
Eventually, Mr. DeAngelis made contractual commitments to sell the oil, collecting fees upfront. He assumes oil prices would continue to rise, but they collapsed. Counterparties wanted the oil, and discovered it was nothing but seawater. The loans were called, and their was no collateral. But the loans were further guaranteed by American Express.
Two days later, JFK was killed in Dallas and the stock market collapsed, forcing a temporary closure of the market. In the coming week, stocks rose but Amex did not and this caught the attention of a small and unknown fund manager. Indeed, Amex was off 50% and drifted lower despite the overall increase in the market in the weeks after JFK was killed. And, of course, the issue was whether Amex’s brand was damaged by their association with De Angelis ( not to mention the financial hit to the company. The guarantees were $175mm, which was a substantial amount at the time).
So, the fund manager set out to do diligence. He visited local restaurants to see if Amex cards were still being used. He sent others to see if hotels were still receiving and accepting travelers checks. And he concluded that the consumer hadn’t cared at all about the Bayonne oil scandal, if they knew about it at all. There was no change in the use of their product. And he knew Amex had the financial wherewithal to make good on its $175mm of loan guarantees So he set to buying stock hand over fist, as much as he could without disrupting the price, He ended up making a killing on Amex.
That is the diligence, in essence, conducted by Lynch. Lynch visited malls to see traffic patterns, warehouses to see shipments and capacity. Yes, he was interested in the products, but more to see what was selling than to understand the actual product. He wanted to understand the business, and the trends.
Which leads to my final point in this lengthy post. Investors don’t need to understand the product or service, per se, as the developer of each would need to. You don’t need to know the algorithm behind google’s search engine to understand the nearly impenetrable advantage it has in search and tailored advertising. But you’d better be able to understand why the business will continue to grow and fend off competition. What’s its edge? Most business earn average rates of return on capital, and their only edge is to improve efficiency and lower costs. It’s the rare business that can earn a return on capital exceeding its cost of capital, and you do need to understand why it does to have conviction. Conviction isn’t needed in times like today, Everyone is a genius because everything just goes up. But when prices fall, and eventually they will, perhaps substantially, how will you have conviction to hold or buy more? Easy to say when everything is rosy. Harder to do when skies turn dark. When it’s time to buy, you won’t want to. That’s why you need to understand the business, it’s value drivers and what is relevant and what is not,
Just as Buffett did when he loaded up on Amex in 1963 after what’s come to be known as the Salad Oil Scandal.