ethers.Wallet.fromMnemonic(mnemonic) generate only one account
You can read from hd wallet Bip32
The Bitcoin reference client uses randomly generated keys. In order to
avoid the necessity for a backup after every transaction, (by default)
100 keys are cached in a pool of reserve keys. Still, these wallets
are not intended to be shared and used on several systems
simultaneously. They support hiding their private keys by using the
wallet encrypt feature and not sharing the password, but such
"neutered" wallets lose the power to generate public keys as well.
I think this is the most important advantage:
Deterministic wallets do not require such frequent backups, and
elliptic curve mathematics permit schemes where one can calculate the
public keys without revealing the private keys. This permits for
example a webshop business to let its webserver generate fresh
addresses (public key hashes) for each order or for each customer,
without giving the webserver access to the corresponding private keys
(which are required for spending the received funds).
However, deterministic wallets typically consist of a single "chain"
of keypairs. The fact that there is only one chain means that sharing
a wallet happens on an all-or-nothing basis. However, in some cases
one only wants some (public) keys to be shared and recoverable. In the
example of a webshop, the webserver does not need access to all public
keys of the merchant's wallet; only to those addresses which are used
to receive customer's payments, and not for example the change
addresses that are generated when the merchant spends money.
Hierarchical deterministic wallets allow such selective sharing by
supporting multiple keypair chains, derived from a single root.
There are further improvements BIP44,BIP49, BIP84. you can read