I have rephrase this section which wasn't exactly crystal clear.
Basically this is a convenient way to access to expression stored in an old cloud expression, while having the new version of the paclet. It's also required to migrate data.
Most user should just migrate and use the new paclet. The migration function is doing a backup of the expression in a file during the process, but one may want to do an additional backup using LegacyGetCloudExpression. Also, one may just check the value of the cloud expression see if it's worth migrating it, or delete it.