The last line of chapter nine is the one that has had a political career: withdraw when the work is done, for that is the way of heaven.
In the context in which the book was assembled this was not abstract advice. A minister who had served a ruler successfully was in a precarious position, since a subordinate with proven ability and accumulated credit is a threat, and the histories of the period are full of officials who won a war and were executed shortly afterwards. The counsel to leave at the moment of success was a survival strategy with a body count behind it.
The line was taken that way for centuries, and the figure of the official who resigns at the peak of his career and disappears into private life became a recognised ideal in Chinese political culture, with the Tao Te Ching as its standard citation.
The philosophical version is broader. An action is not complete when its object is achieved; it is complete when the actor stops. Continuing past that point turns the achievement into a position that has to be defended, which is a different and worse situation than the one the action was meant to produce. Chapter two says the same thing about merit that is not dwelt on, and chapter thirty says it about a general who achieves the result and does not press on.
It is also one of the few pieces of advice in the book that can be followed directly, which may be why it stuck.