Addition to SHS...?

Hi,

Just a quick question...

<snip from NIST>
NIST is proposing a change notice for FIPS 180-2, the Secure Hash Standard
that will specify an additional hash function, SHA-224, that is based on
SHA-256. The change notice is available at
http://csrc.nist.gov/publications/drafts.html. NIST requests comments for
the change notice by January 16, 2004.
<snip>

What the reason for specifying a 224? - Obviously one could think of
computation speed, but since it's based on 256?

Kind regards
Kim