Salary mainly. You may keep the job if you're tenured but without the grants, you may not get much in the way of salary. Also, your grad students, postdocs, and resources (lab equipment, computing time, etc) are being paid through grant money so without it you don't have a lab group. Finally, without funding, your department and/or the university can make life exceedingly difficult. Extra teaching, more admin tasks, etc.
It is common for the university to pay a partial salary (9-months) and then the faculty can pay themselves another 3 months from grants if they have the funding. At University of Michigan the 9-month salary is spread out over 12 months, but you get a nice summer bonus if you have grant money.
The publicly listed listed salaries are the 9-month salaries, so faculty pay isn't quite as bad as it looks at first glance. If you have funding, most grants also REQUIRE the professor to take salary money from it, otherwise they assume you aren't actually putting in any effort on that funded project.
You cannot be fired with tenure, which is nice. But you can still move up and down in status. The grants provide supplemental income. You can do work you love enough to take a lower wage than you probably could get in the private sector. There are benefits.
A friend of mine got on it after tenure (art school) because he could finance cool projects of his own design that way.
But even in a research university isn’t tenure supposed to protect you from being pushed around by the administration?