They are de facto semantic major versions - think of recent-ish additions like f-strings and match-case (3.7 and 3.11, I think), you'd get a syntax error in an older parser. PyPy targeting 3.9 for example would would support f-strings but not match-case.

Or at runtime, you can import things from the standard library which require a minimum 3.x. - .x releases frequently if not always add things, or even change an existing API.

For Python, 0.1 increases are major versions and 1.0 increases are cataclysmic shifts.