Every single piece of security software you run has to try force security updates, otherwise you could be running a trivially compromised piece of software. Most closed-source software, and even open-source software that isn't as sensitive to tampering as Monero, use things like mailing lists to notify you of a new major version. Lots of them also use in-app notifications, which users have become adept at ignoring or clicking away.

Having regular, timed hard forks forces users to update the very sensitive piece of security software they're running, and benefit from all the patches, bug fixes, and reduced exploit vectors in newer versions. Being able to add functionality, and know that the whole network has access to it in-step with each other, is just icing on the cake.

In future when Monero is more stable we'll go down to annual hard forks, as mentioned elsewhere in these answers. If the core Monero daemon gets to a point where it's particularly secure and particularly stable (eg. the code has been formally verified, and been in production for years with little or no incidences) we can even go stretch them out further.