If you want to know why the question was downvoted, of course the obligatory answer is you can never know. You can guess, or make educated guesses. We can certainly point out problems in the post that may have contributed to it being downvoted, but we can't know.
Moving on, we can start by looking at the comments on that question. There are two that indicate problems with the question. Both are likely indicating reasons for downvoting.
To be specific, the first is indicating that the question doesn't demonstrate what the author tried. The second is saying that there are lots of existing solutions widely available/accessible, and that the author didn't do his research in finding them. However, those comments didn't state that explicitly. Rather than just explaining what the post did wrong, from a critical perspective, they are phased as to how the author should fix the problems, indicating that he should be explaining what he has tried and why the existing solutions that are readily accessible do not work for him. This is great, as this phrasing will lead the author to edit the question to improve it (which he has since done to at least partially address those points) rather than getting into a debate with the commentors over why his post isn't bad.
As for "is it necessary to downvote it" yes. Downvoting the question serves several purposes. It indicates to the author and to other possible readers of the question that the question is not well researched and/or is not useful. It incentives the author to fix up the question, as well as being an indication that there is even something to fix up in the first place. It also means that if the author doesn't fix up the post he needs to deal with the loss of reputation, as well as the fact that the post can contribute to a question ban if there is a pattern of posting very low quality content. Downvotes are a major (although not only) signal to this automated process that content is of low quality.
Downvotes also feed several automated deletion scripts which can delete content of sufficiently low quality after a period of time.