Primarily, public-folder connection agreements (CAs) synchronize the mail addresses of Exchange Server 5.5 public folders to Active Directory (AD) and, conversely, the mail addresses of Exchange 2000 Server public folders to the Exchange Server 5.5 Directory Service (DS). However, you need to use one two-way public-folder CA per Exchange Server 5.5 site for other reasons, too.
If you attempt to administer a public folder created on Exchange Server 5.5 from the Microsoft Management Console (MMC) Exchange System Manager snap-in and you haven't created an entry in AD for that public folder with a public folder CA, Exchange System Manager will generate errors signifying that it can't access the object. You see these errors because the properties of the Exchange Server 5.5 public folder show that the folder is mail-enabled and Exchange System Manager attempts to look up the address properties in AD. Exchange System Manager can't retrieve the properties if the object hasn't been synchronized to AD and thus doesn't exist.
Similarly, in Exchange Server 5.5, the Microsoft Exchange Administrator program expects that any public folders it accesses have mail-address properties because this behavior is the default with Exchange Server 5.5 public folders. Accordingly, if you haven't implemented a public-folder CA to synchronize mail addresses from an Exchange 2000 public folder into the Exchange Server 5.5 DS, you can't administer such an Exchange 2000 public folder with Exchange Administrator.
Because Exchange Server 5.5 always expects to have email addresses associated with public folders, running a DS/Information Store (DS/IS) consistency checker on Exchange Server 5.5 can cause serious problems. The Exchange Server 5.5 Public Folder Store will have knowledge of Exchange 2000 public folders (because the public folder hierarchy is replicated by default), but if the Public Folder Store doesn't find an associated Exchange Server 5.5 DS entry for that public folder, it will generate an entry. Effectively, this action results in two separate email addresses for the same public folder. Introducing a public-folder CA in the future can then result in two separate directory entries (in both the Exchange Server 5.5 DS and AD), which would prevent the public folder from receiving mail.