The real reason why such policies are in place is because they are in place by default. That's how things go in Active Directory:
- Passwords expire after 42 days.
- When changing his password, the (non-admin) user cannot reuse one of his 24 previous passwords.
- User cannot change his password twice within the same 24-hour frame.
So you will encounter such things a lot, mostly because it would require efforts and understanding to set them otherwise. Most people go through their life in a state of blissful ignorance and laziness, and sysadmins are no exception.
When a rationale for the third property (24 hours between password changes) is needed, the oft-cited reason is what @bobince says: to prevent a snarky user from cycling through 23 dummy passwords to get back at his initial password, because that would contradict the first rule (no password reuse).
Of course, such rules won't prevent users from using "sequence passwords": Password37, Password38, Password39... which somehow defeats the purpose of forcing password expiry (purpose which is already of very dubious value). And preventing the user from changing his password as often as he wants also means that the user cannot change his password as often as he needs: if the user notices a shoulder surfer who just stole his password, a security aware user would like to quickly change his own password, which would be, in that case, a very good idea. The rule against password change may prevent that.