Certification in general is good as "filler" on a resume as a substitute for experience, and might help get you past the initial screening process.
Certification is not a good substitute for actual experience. So if you are deciding between taking an entry level job in your field (gaining experience) and holding off on a job and focusing on collecting some certifications instead, take the job. Every time.
If you're stuck in an entry level job and your employer is willing to pay for certifications as some kind of ersatz job advancement -- take advantage of it. You might learn something, and it can't hurt.
If you want to transition from one job (say helpdesk) to another (say networking, or security) then pursuing a certification on your own time is a good indicator to your current or future employer that you have an interest and might be a good hire in the different role.
If you're deciding between a 4-year degree and some kind of sequence of advanced certifications -- do the degree. A bachelors degree, even in an unrelated field, carries far more weight with HR departments than a bunch of obscure IT certifications.
Often this is a chicken-and-egg problem -- you can't get a job without experience, and you can't get experience without a job. Sometimes certification can help get over this hurdle.
If you are mid-career and already employed at or near the top of your range, certification is unlikely to be a gateway to advancing your career. However, pursuing personal interests and "having something to show for it" might be worth it. Especially if your employer will fund it...
If you have significant experience with a unique system -- For example, a specific medical record system or manufacturing control system -- getting certified by that vendor in that product may open doors for future contract work. Note that this would not apply to horizontal certifications like Microsoft or Cisco -- while those certifications have their own uses, they are generally too broad. I am talking about the company who needs an expert with XYZ system to keep on retainer -- these people can command a comfortable hourly rate.
Finally, if you work for, or hope to work for, one of the consultancies that hire you out for short term contracts -- one where they have to sell you over and over again to potential clients -- then having an exhaustive list of certifications might be helpful or even required.
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