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As a margin of error.

"this tree was planted 200 years ago, +/- 10 years" -> the tree was planted between 190 and 210 years ago.

"this comet will next be seen in 100 years, +/- 10" -> The comet will visible between 90 and 110 years from now.

Yeah, interesting. In The Netherlands it is commonly used to mean "more or less", or "approximately", unless it is used in formulas.

For “approximately 10 years” I would note it as ~10. Native Australian English speaker.

In English, "give or take".