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To elaborate further, you can think of chords in one of two ways. The first would be structural chords, which begin and end phrases (hence, they are found at cadences). The second would be prolongational chords, which embellish or prolong structural chords, delaying the cadence until an appropriate moment. Normally (in the Classical style), I and vi would be considered tonic function, V and vii dominant function, and IV and ii predominant function. Because I and vi are from the same category in your example, they are an instance of a tonic prolongation (there is no change in function, no effective harmonic motion, and therefore no cadence). Other examples would be: I IV I (where the IV chord acts not as a predominant but rather as a neighboring chord) and I V I at the beginning of a phrase (not the cadence), where the V is usually inverted. Like Phrygian Queen suggested, though, function is not always tied to scale degrees. For instance, there are passages where, say, V is the prevailing harmony and I acts as a neighboring chord to the V (much like the I IV I progression discussed above) - a dominant prolongation. Other chords may serve as prolongational chords, as well.