Some of what 1password provides is a synergy of security and convenience.
1password's browser integration can save you from phishing, because it can tell g00gle.com from google.com every time and not offer to fill in your google.com login, whereas if you're used to just copying from your password manager, you have to make sure not to mess that up.
Lack of convenience means manual work, which means chance for error. This is the same reason we recommend coding using trustworthy high-level crypto constructs instead of implementing it yourself; it's more convenient and harder to mess up.
1password's browser integration can save you from phishing, because it can tell g00gle.com from google.com every time and not offer to fill in your google.com login, whereas if you're used to just copying from your password manager, you have to make sure not to mess that up.
Lack of convenience means manual work, which means chance for error. This is the same reason we recommend coding using trustworthy high-level crypto constructs instead of implementing it yourself; it's more convenient and harder to mess up.