If their otherwise random attacks on the cipher managed to sometimes produce those words or (preferably) phrases, they would know they might be on the right track.
When those words or phrases appeared, they would feed the settings they had used to reveal them back into the whole encrypted message to good effect.
Due to the regimented style of military reports, it would contain the word Wetter (German for "weather") at the same location in every message.
[4] At Bletchley Park in World War II, strenuous efforts were made to use (and even force the Germans to produce) messages with known plaintext.
For example, when cribs were lacking, Bletchley Park would sometimes ask the Royal Air Force to "seed" a particular area in the North Sea with mines (a process that came to be known as gardening, by obvious reference).
These devices were immune to known-plaintext attack; however, they were point-to-point links and required massive supplies of one-time tapes.