Be “just okay” doing many things or hone your skills and focus, and be amazing at one thing: I know which one I would choose. There are many examples in life we can choose from which highlight the benefits of being a specialist over a generalist.
The medical industry is a great example of generalisation and specialisation. A general practitioner is a generalist, a nurse is a generalist. They both can do many things, but if they want to increase their earning potential they need to specialise.
Another example is a dentist vs an orthodontist. A dentist can do basic dental procedures and generally make a good living, but there are some things a dentist cannot do and one of those things is straightening teeth. If you want to get braces or other medical treatments involving straightening of teeth, only a specialist orthodontist can do that.
More often than not, generalists get paid less than specialists and this is not only apparent in the medical industry, but also development. A developer who specialises in machine learning and/or data science is going to be paid more than a generalist developer who builds Wordpress websites and also builds HTML newsletters.
When companies advertise for generalists, to me it is a sign that companies are looking to get more for less. Having said that, ever specialist I have worked with or know of (myself included) has knowledge of other areas because some things have overlap, but largely the front-end for myself is where I know the most.