Sharing password between different services is bad information security hygiene. You would need to keep track of all the data breaches going on around the world - and when one does inevitably happen, begins the nightmare of updating said password everywhere you’ve been using it.
Using variations of a main password doesn’t work either. If your variation technique is simple enough to be easy to remember, it’s easy enough for a computer to guess.
Of course, no one expects humans to remember a hundred different passwords. The solution is to generate random passwords and have them stored somewhere: your password manager. You have to remember a single password, the one that unlocks your vault of secrets, and you can trust your password manager to remember the rest.
But having a single all-powerful password isn’t just as dangerous as using this password everywhere? Apart from the obvious logistic advantage of having a single place to change this password if it ever becomes compromised, password managers offer something that most services don’t: multi-factor authentication (MFA).
You can configure your password manager to unlock with fingerprints, require phone confirmation and whatever else you feel necessary to ensure that even if your master password falls in the wrong hands, accessing your crucial services won’t be as simple as entering your login details on a web page.
Also, most password managers will offer a whole collection of useful utilities that go along great with securing passwords. They monitor data breaches in services you have accounts, they can notify you in case one of your passwords ever enters the public domain of password breaches known and loved by hackers, they can store non-password but secret details like documents and credit card information so it’s easier but secure to fill forms online, share passwords with select groups of people and so on.