In the 80s and 90s, some desktop PCs included a "Turbo" button, but what it actually did was a little unclear. What was it for and why don't PCs have them now?

When the H-11 came out in 1977, Heathkit called it a “personal computer”. There’s circular logic here. The VIC-20 and C-64 didn’t have turbo buttons, neither did the Atari ET or the Amiga 500.