How will certification help in a testing career and up to what extent will it help?
According to my experience does not matter whether you certified or not. The knowledge and experience is important you have.
On the other hand, I have to say that I have ISTQB CTFL and CTAL-TM certification and I'm going to do the others as well because of the knowledge. The fact that I have these certifications my existence as tester has some strength. The knowledge and the experience I have is not questioned as many times as my colleagues' skills are questioned. On the other hand our clients are satisfied when they can see that there is a partner who has employees having these certifications.
But, I should write with capitals letter this but, these certifications do not give you knowledge, experience and humility which is required in the testing world. They are not equivalent with the willing to learn attitude.
Here is another aspect of having certifications. You will be in focus. There will be people who watching you as a example.
Is certification courses like ISTQB necessary in testing career
It's clearly not necessary, as the many non-certified testers can attest. I have been in this business a long time, and have never felt the need to become certified. And I have hired a lot of people - never once requiring or caring if they were certified.
In my experience, for the certified, it's not actually the certification that is useful, but the learning that is actually useful.
In my opinion, you can gain all the knowledge you need on your own, or you can go spend money on certification courses and gain some knowledge (which may or may not actually be useful/needed).
A while back, I performed a quick experiment to see if certifications were actually useful in getting hired. My conclusion - "So will having a CSTE or CSQA help you get hired? Perhaps. But it probably won't matter."
You can try a similar experiment yourself: Software QA Certifications - An Aid To Gaining Employment?