Backup
For both types of wallet, legacy or descriptor, the type of backup provided by the developers is to back up the whole of the wallet.dat file. If you just back up only the private keys you lose information needed to efficiently reconstruct the wallet.
Backups and “future” transactions
Restoring an old wallet backup made years ago, and allowing the wallet software to resynchronise (“catch up” to the blockchain) will allow that wallet to discover and record transactions made after the backup was made. This applies to both legacy and descriptor wallets.
Descriptors
There are likely to be some good answers to existing questions here that explain the motivation and practical differences between legacy and descriptor wallets. However, I believe the main intent is to record not just the raw key data, but also information about what types of output script have been generated and are to be generated from those keys. You can think of these different patterns of script as address types. There has been some proliferation of types of output script, which I believe makes it more useful to use descriptors to keep track of this aspect of key usage.
