It's a recommended practice (I hate the term "best", everything depends).
Why? Quite easy actually - having random passwords is better than reusing the same everywhere. Random passwords are impossible to remember by a regular human, hence you need a password manager. Using a local file as a password manager poses a usability/availability risk (you have to sync it yourself, you have to back it up yourself, you have to make it available on all devices without putting it at risk, you have to secure it, etc.), hence cloud-based password managers are better for the average person, especially coupled with MFA for critical accounts (banks, email, etc.). If you're a highly technical or highly security conscious person, or under threat, the equation changes of course, but the recommendation for a cloud-based password manager isn't meant to apply to everyone, just most people.
Why? Quite easy actually - having random passwords is better than reusing the same everywhere. Random passwords are impossible to remember by a regular human, hence you need a password manager. Using a local file as a password manager poses a usability/availability risk (you have to sync it yourself, you have to back it up yourself, you have to make it available on all devices without putting it at risk, you have to secure it, etc.), hence cloud-based password managers are better for the average person, especially coupled with MFA for critical accounts (banks, email, etc.). If you're a highly technical or highly security conscious person, or under threat, the equation changes of course, but the recommendation for a cloud-based password manager isn't meant to apply to everyone, just most people.