If you look at the assigned identifiers in the binary scheme, the authors believe that 32-bit truncated cryptographic hash functions are useful.
No they don't. From Section 2:
The sha-256 algorithm as specified in [SHA-256] is mandatory to
implement; that is, implementations MUST be able to generate/send and
to accept/process names based on a sha-256 hash. However,
implementations MAY support additional hash algorithms and MAY use
those for specific names, for example, in a constrained environment
where sha-256 is non-optimal or where truncated names are needed to
fit into corresponding protocols (when a higher collision probability
can be tolerated).
Truncated hashes MAY be supported. When a hash value is truncated,
the name MUST indicate this. Therefore, we use different hash
algorithm strings in these cases, such as sha-256-32 for a 32-bit
truncation of a sha-256 output. A 32-bit truncated hash is
essentially useless for security in almost all cases but might be
useful for naming. With current best practices [RFC3766], very few,
if any, applications making use of names with less than 100-bit
hashes will have useful security properties.