Composers know that the outline of a melodic line can be reproduced at a higher or lower pitch to help bring out all the life and beauty that's inherent within it.
This is called a sequence.
Sequencing is very commonly used to help create new material for the extension of a work, and it sounds very natural.
Sequencing is in fact involved with the replication of DNA in the reproduction process of all natural life as we know it (photo).
Not every piece of music has sequences written into it, but many do; and they lend interest to any passage that has them.
Any of the larger musical forms require the repetition of material in different keys, and sequences play a large role in that.
This is yet another time-honored compositional tool that can be applied to virtually any style of music, but it comes in particularly handy in working the episodes of fugues.
For some examples of sequences and how they work, the scores of the Fugues from Op. 6, 10, 13, 18, 19, 20, and 21 posted on this web site may be consulted [See menu bar, Free Stuff].
The d minor Fugue from Op. 10, for example, is loaded with those; they're not only written into the episodes (modulating passages between entries) but also into the exposition.
Sequences, if not overdone, can be very effective.
There are also sequences in the development sections of the Op. 5 and Op. 25 Preludes which will repay careful study.