While the Raspberry Pi has very good support for an I2C bus, a lot of very cool chips – including the in system programmer for just about every ATtiny and ATmega microcontroller – use an SPI bus.

The "SPI Flash Market by Technologies (3D NAND, EEPROM, NAND), Interface (Concurrent, Parallel, Serial (SPI)), Programming Methods, End-User Industries, Applications - Global Forecast 2025-2030" ...

One Programming: Unified SPI and AOI inspection program data eliminates redundant setup and programming tasks. Easy Programming: Simplified and automated program creation minimizes operator ...