I personally stand for as little blackboxing as possible. So I'd say it's very important.

You have to know how your language behaves under most conditions to generate "optimal" code and be able to assert certain asumptions you will make.

classic for example is the reference behaviour in js for complex structure in combination with the event loop or in php my alltime fav ... the datetime object ... references can be a bitch.

you have to know your tools to waste less time on problems arising there and you can use multiple selfcontaining technologies.

For example knowing how: tsc/babel/webpack/gulp/grunt/phing/pip/npm/maven/composer/ant do work will allow you to keep the build system close to the language, so you can separate the concerns of the setup and keep it simpler.

Every node guy will know how node works but if I trigger explicit "./node_modules/.bin/bla" commands from ant or phing via commandline they will be confused because they're not used to look in the build.xml because it's not the natural place to look at.

These are basic examples, but I think that's why you should take your time to get to know your tools.

Frameworks are the same so you can actually decide, if using it makes sense in a certain context.

Should surgeon know the medicine and anatomy before doing operations?

Will you go to a dentist and allow him to work on your own teeth if he has no understanding on "internals" about your teeth?

Will you register on a website and put your password in the DB knowing that it was developed by a person without a necessary knowledge?

I'm sure you know the answer to your question and let me underline the main part of it - "on a daily basis".

And if there is no practice (knowledge) then you can't fight (apply) it in a right direction.

Depends how deep you're talking about. You should certainly understand how it works; and hence its strengths and weaknesses. But do you need to know the deepest darkest guts of its code? Not really.

To break down an example, take good ol' jQuery: you should know it uses Sizzle, creates its own element collections, understand how it traverses the DOM, etc. That level of knowledge is critical because it impacts the code you write on top of it. If you prefer the new and shiny version, you need to understand React's virtual DOM, etc...

But do you need to have read its source code line by line and understand every last trick and nuance before you use it? No. It's a great learning experience and you definitely shouldn't be afraid to dive in there and take a look, but it's not required to write good code.

It's interesting to know the tool you use in a daily basis, and know all the potential. And if you want to deep a bit in the internals you can discover new and interesting things that can help you in your development.