But an external HDD is not going to sync to a central server or other devices. icloud/onedrive etc is not just the space but also redundancy and backup and sync bultiin.

Value here is not just the amount of storage:

1. Multiple people and devices use this one account.

2. Said multiple people are not technical - they don’t always know/remember to plug in a drive to their device for backup.

3. The mental load reduction is amazing. The iCloud keychain feature + faceID + multidevices is so convenient, I don’t understand why I ever spent effort in crazy tool and file plumbing schemes. I use the $2.99/month plan, and my time is worth a lot more than that (at least 3000 times more using paycheck…). I’d rather spend time with friends and family doing other things than debugging our collective backup schemes. That’s value.

you're not paying for the hard drive, otherwise they'd ship you the hard drive - you're paying for the management.