The selling point of a Steam Deck is that it is a PC console. One of the problems with the PC gaming market is that even though in theory all PCs are "compatible", the reality for game developers is very different. Different PCs have subtly (or not so subtly) incompatible software and hardware, and it's up to OS developers to try to maintain some semblance of compatibility, and game developers to ensure that their product runs on as many configurations as possible.
The problem is that there is a large market segment that would enjoy the greater variety of games that are available on PC than on any given console, but they simply view a gaming PC as another "console". I have a friend who I would consider a "gamer" (a long history of console ownership, hundreds of hours logged on his Nintendo Switch on "hardcore" games), whose only PC is a laptop that is too weak to run any modern games, and feels that buying another appliance just to play a handful of games he can't currently access doesn't make sense.
The Steam Deck bridges the gap by providing a console experience for PC games. Developers only need target one hardware and software configuration to ensure that any Steam Deck owner can play their games. The Steam Deck operating system indicates which games run well, and provides out-of-the-box settings for controller and graphics configurations that ensure that a Steam Deck owner can buy a game and be reasonably sure that they won't have to spend any time updating graphics drivers, remapping controls, tweaking settings, or troubleshooting PC-centric issues just so they can play a PC game. It inhabits a handheld form factor because that is the best selling form factor (see Game Boy, Nintendo DS, etc.) with the added bonus that it can be docked and played like a regular console. The same combination that propelled the Nintendo Switch to massive success.
People outside the HN echo chamber don't care about the arcane hardware and software issues that cause many to turn away in disgust, they just want to buy a device that gives them access to a library of games they wouldn't otherwise be able to play. At present, the Steam Deck is the device that does that the best.
