In case you want to look up something critical (airline flight information?) when there happens to be a 30-minute GMail outage?

In case you want to take your data with you when you migrate to a different service?

In case you want an archive of your correspondence that will live on even if Google goes out of business decades from now?

In case you want to keep snapshots of your mail folders and be able to compare them, to see when a message arrived or was moved or deleted?

So you can access your entire mail archive when you're offline?