I am a die hard fan of chopins music but I fail to see the merit in some of his preludes. He's got the market cornered on nocturnes, walzes, ballades and so on...but what of the preludes. Some of them seem to be composed just for the sake of being composed. Enlighten me please!!! Mark 
If you can (as Allchopin said) supply specific preludes, we can discuss them in more detail.
In general:
Chopin’s preludes are amongst the most profound of his compositions – if not the most profound. They were composed quite late in his life and were not “salon” pieces as many of his waltzes, mazurkas and nocturnes. I have the impression that he might have been growing dissatisfied with his more “commercial” and easy listening production, and decide to do something really different where no concession at all were made. He took advantage of the free-form of the prelude to expand his compositional output. They are obviously an homage to J.S. Bach 48, a composer he greatly admired, hence 24 preludes one in each key.
They are unique in several aspects. They were published as single collection (no other of Chopin’s works apart from the studies was published in this form), which gives credence to Claudio Arrau’s (and others) view that they were a cycle and frequently played them in concert without any break between them. I think there is great merit in this idea. (Chopin himself never played more than 3 or 4 in concert – but audiences were quite different then).
Eleanor Bailie describes them as “starkly epigrammatic ejaculations of fury and violence, besides moments of thistledown lightness of spirit, of incorporeality, grandeur or grinding despair”.
But perhaps most interestingly is how much they depart from the expected. If these preludes were to appear from nowhere, would we say they are by Chopin? Would we even be able to correctly identify the period? Harmonically they are amazingly modern, and although each one concentrates on a single musical idea, each one explores the totality of feeling, shade and colour in that idea.
Perhaps the greatest difficulty in playing them is how short they are: the performer must capture instantly the mood and spirit of each one. My favourite recordings of them are by Claudio Arrau (Phillips), Martha Argerich (DG and Ivo Pogorelich (DG). Their interpretations are very individual, and this can give you an idea of how much interpretative room there is in these miniatures.
I now have grown tired of Chopin’s Waltzes, Mazurkas and Nocturnes. Yet I can still listen to these preludes and feel like I am listening to them for the first time. Schumann called them “Eagle’s wings” and right he was.
If you can’t see what the whole fuss is all about, play them. Most of them are not technically difficult (although some require the utmost virtuosity – no. 5, 12, 16, 18, 19 and 24). Musically they are all infinite in their possibilities. My favourites amongst all of Chopin’s pieces.

Best wishes,
Bernhard.