Some hashing algorithms have a configurable output length. You need to encode the length, or have it as part of the type, and it's more uniform to just have it separate from the type.
I guess it depends on how precious space is. A naked hash is very information-dense. In certain applications, inserting a prefix of several bytes to each hash makes a difference. OTOH, if the hashes end up being inserted into a table in MySQL, then space is probably not that precious.