By design, GAAP accounting principles are very conservative, to make it harder for deceptive actors to conceal losses. However, for making management decisions, GAAP accounting principles are so conservative and rigid that they can actually obscure the key drivers of a business. This is why management accounting is historically done differently than financial accounting. Financial accounting is optimized for revealing fraud, not for understanding or running businesses. Therefore, if someone chooses to use non-GAAP accounting, you're right it can mean they are hiding something, but it can just as easily mean they are trying to relay more useful and informative numbers.

In all cases, of course, it's important to understand what the numbers mean.

In Tesla case, it's because the non-GAAP numbers represent how they view and make decisions about their business internally. It makes them look better in some dimensions and worse in others. They report on it because it gives insights into how they think about things.

Based on the analyses I've read, it looks like a big driver of the non-GAAP difference is Tesla completely ignoring the cost of acquiring SolarCity. Is that a fair analysis? It depends. Those non-GAAP numbers suggest the car manufacturing business is doing well, but hide how Tesla is doing overall (including cash flow) as a result of SolarCity.