It materially encouraged you to keep your 3DS on you while you were out and about, which gave the system a lot of visibility in its shaky early years. It made 3DS players feel more like a community, and gave you another place to show off your Miis.
I understand why Nintendo removed it - the Switch straddles the line between home console and handheld, and StreetPass features would discourage leaving your system docked as a home console - plus, the Switch is just a lot larger and more cumbersome to carry around. But still, I can't help but feel that Nintendo cut all of the features of the last generation which actually gave their systems personality: Miiverse, system themes, StreetPass, Miis, etc., and we're poorer for it. Hell, we still don't even have custom themes on the Switch after like 7 years!
Whatever the successor to the Switch is, I hope Nintendo starts experimenting with these software value-adds again, since they can be the glue that holds people to a platform. These days, if a game is identical across the Switch and PC aside from worse performance on the Switch, I'm getting it on my Steam Deck instead. But if the Switch version actually used features unique to a Nintendo platform again, I would be much more willing to consider it.

