If by "downloads available" you mean a OS upgrade, that is because those are not official releases, and since Kies is an official Samsung application they do not provide unofficial releases.
Currently, the last official release for the Samsung Galaxy S is Android 2.2 Froyo.
If you are referring to "official releases" and your OS is < 2.2, then you can have some variant system, for which there is no firmware upgrade, or you can have some problems with a not-up-to-date Kies version.
