In this case, the snapshot is being done to support the release of a new client and IRI, which will contain a number of fixes and optimizations. These reference a new database structure. Although the devs have not said yet, this new client will likely not be backward compatible. But the snapshot preserves all of the current balances and addresses.

You do not need to be logged in. In fact, you will need to download the new version of the wallet and IRI, which means you will have to log out anyway.