keypool option with HD wallet I'm a bit confused.

According to https://bitcoincore.org/en/2018/02/26/release-0.16.0/#hd-wallets-by-default HD wallet is enabled by default. However there is still keypool option of bitcoind which sets how many addresses will be pregenerated on wallet loading. keypool assumes regular wallet backups in order to not to loose keys generated above keypool value and keypool restore.

So, my question is: with HD wallets does keypool option have any meaning? And do I still need to do these regular backups?
The keypool is how Bitcoin Core achieves the lookahead functionality when you restore a backup. It is effectively the "gap limit". While BIP 32 does mean that one backup should be sufficient, it is possible that the keypool is too small (if you somehow hand out thousands of addresses but never receive at them) so backing up routinely is still a good thing to do. Also, backing up routinely allows you to keep metadata like labels when you restore.

The keypool is also useful for encrypted wallets as it allows getting new addresses to give out without having to unlock the wallet every time.