Each of the three sentences is a combination of two statements that might have been made independently. The same principle of comma use applies to participial phrases and to appositives.
Understanding is that penetrating quality of knowledge that grows from theory, practice, conviction, assertion, error, and humiliation.
A comparison of the three forms given above will show clearly the advantage of the first.
It is, at least in the examples given, better than the second form because it suggests the close relationship between the two statements in a way that the second does not attempt, and better than the third because it is briefer and therefore more forcible.
The colon also has certain functions of form: to follow the salutation of a formal letter, to separate hour from minute in a notation of time, and to separate the title of a work from its subtitle or a Bible chapter from a verse.
His first thought on getting out of bed — if he had any thought at all — was to get back in again.