In calculus when dealing with a limit at 1, it is appropriate to sometimes treat 0.9999... as 1 because a function can get infinitely close to 1, but as long as that limit's there, it will never fully achieve 1. But we still need to occasionally do work with 1, so we substitute 0.9999... instead.
However if we look at this from a chaotician's perspective, differences in starting conditions may lead to enormous differences in a system farther down the line, (source: Jurassic Park) therefore treating 0.9999... and 1 as equal can be dangerous.
It is all about context.