It depends. For vendor exams, I usually take the test rubric and fill it out with the vendor’s documentation. For exams that aren’t associated with a vendor, I still take the test rubric and fill it out after doing some research on each subject area.
After this, I go take the test. If I pass, great, if not, I study the test rubric that I just made into a study guide.
I also only take exams strategically, either it’s required for a job, or I find the most valuable certification in a field I’m interested in pivoting into. Very early on, I thought that collecting acronyms was a good thing, but you learn quickly that it’s not. Rajesh collects acronyms, Rajesh has an email signature that’s larger than the message in the email body. Rajesh doesn’t know anything. Rajesh is the reason why the H1-B visa should be eliminated or reduced in scope to graduates of American universities only. Do not be like Rajesh.
If you can afford it, you’ve been in the industry for awhile, and have a decent understanding of the concepts and tech in the exam, taking the exam to get a benchmark of where you are is also a good idea in my opinion. You use an exam attempt and it costs money, but you can really focus on the areas you didn’t do great on while studying to take it again and it gives you some direction.