Charles Rosen, in a chapter from his book "Freedom and the Arts" titled Tradition without Convention, recounts a time in his practice room when, while working on a passage from one of Mozart's piano concertos, he accidentally strayed into a different work by the same composer.
This leads Rosen into a discussion of these kinds of virtuosic but often formulaic passages in much late 18th century music. The value of these passages is in the structural role they play in the work; joining contrasting sections in a relatively seamless way. Hence Rosen's opinion that most modern scholars under represent the importance of these passages; they play a vital, structural role.
As forms become longer in duration, these "filler" passages did much to extend works' length. More importantly, Rosen states, extended "filler" passages gave more power to the half and full cadences in the work, allowing the composer to raise "the long-range power of the harmony to a higher level." Conventional passages made these larger forms more intelligible. "They made it possible for the audience to admire the performers without having to strain to admire the composition."
The aesthetics of the late 18th-century came to regard the conventional as undesirable, an attitude that continues into our day. In 18th-century aesthetics, "the conventional is both commonplace and arbitrary. A convention is accepted by everyone precisely because it is arbitrary, because it is imposed. There can be no disagreement because there is no argument. For the eighteenth-century critic, the signs of painting are "natural": that is, the painted image of a tree signifies a tree because it looks like a tree."
Convention remains as long as we never question it. As soon as it is, it is transformed into something else. Something expressive. The demand for originality began to challenge those conventional passages in Mozart and Haydn. Once they had been called into question, it is only a matter of time before they were reinvented or dispensed with.
Beethoven contains passages that, at first glance, seem the same as those classically conventional ones of Mozart et al. The opening of Beethoven's piano sonata in Ab (opus 110), shown in figure 1, might at first appear to do something similar to what every Mozart piano sonata does: simply and straightforwardly establish the tonic key. While it surely does this, we only need to glance at the beginning to the next piano sonata (see Figure 2) to realise that this tonicisation at the outset is only optional. Beethoven's opening to opus 110 transforms conventional material into much more than that; it becomes profound and expressive.
Figure 1: Opening to Piano Sonata #31 Op. 110
Figure 2: Opening to Piano Sonata #32 Op. 111