As an example, I recently took my SteamDeck on a trip. I played it on both flights, played it when I had some downtime at the hotel, and using a Bluetooth keyboard I got some minor work done using Desktop mode.

Because of this I didn't need to bring a whole laptop with me, saving me space. It lasted longer than my laptop would've playing games, especially when using the external battery pack I have, and it's way more comfortable to use on tight airline seats with tiny tray tables.

For at home, I just often like to handheld game, on a couch or in bed, rather than being tethered to my Desktop or TV. Because of this I've owned almost every generation of Nintendo handhelds. But as I do often enjoy fancier games than what handhelds got in the past, I owned a few consoles, and then eventually transitioned to PC gaming. Steam Deck finally bridged the gap between those two things, giving me the ability to play my PC games on a handheld, and with better performance and flexibility than a Nintendo Switch. Plus PC games often have great sales, and there's a wider selection of indie titles that the Switch doesn't get.