Castellations also enable use in a "burning fixture" like the below. The fixture has bent pins that hold the PCB physically and connect electrically to the PCB's leads.
Would love to see someone design a PLCC or IC Socket that would let you slide one of these in (like those old sockets they had in Macs for optional FPU chips).
I just looked that up and it seems like a nice system (although physically complex/costly to manufacture) [0]. The only FPU-optional Mac I had was a IIsi which had an optional FPU that used the Nubus slot.
https://www.amazon.com/DIYmall-ESP32-WROOM-32-Programming-Fi...