Brands starting doing this in the 486 days to match customer expectation. In the 286 and 386 days it slowed the PC down to 8MHz. It was called turbo because it was 8088 Turbo mode, as the 8088 nominally operated at 4.7MHz.

Source: Those of around since the 8088 days viciously mocked the brands that did this on newsgroups because they were catering to newbs... or whatever we called them back then.