Or do you have to provide your own? How essential are tablets for game development?
Of course tablets are essential for cg artists. Even in the smallest company artist have company (and big) tablets.
I can't imagine company which would ask you for bringing your own, maybe some garage startup...
As a employe you will make money to your company, they will take care about your equipment - so you are optimized and comfortable to make even more money for them :-)
Totally depends on the company. I doubt there's a standard for this, since this would be up to the artists preference. My work provided me with a ergonomic keyboard when I asked for one. So the best you can do is ask when you're hired, or if it's critical to you, make it a condition of your employment.
I am going to try and suggest the underlying question is along the lines of "What technologies are good for an artist who wants to get into game design/development" as opposed to do companies provide tablets.
In general use whatever technologies allow you to do your best work. If that is a tablet drawing surface then so be it. If a company likes your work they will hire you and provide the interface you are most comfortable with.
Now to wrap it around to the originally posted question, the company I previously worked for Did buy tablets for the artists who requested them which was just under half of them. But this did extend to every department with their interfaces. Natural keyboards, mice even displays and audio devices as needed were provided.
If the company feels it will make you more productive and in turn that this enhanced productivity will recoup the cost in a reasonable amount of time they will buy you almost anything job related.