Someone is asking a question about autocomplete options for jQuery. Admittedly the subject line is misleading, but why the -3 downvote for asking such a question? Does it provide value to the person asking the question? Does it provide value to the community? Please help me understand the benefits of downvoting this particular question without providing a comment or context.
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.
The question, especially when originally posted, shows no research effort. Without even Googling I am confident there are autocomplete solutions out there. If the OP hasn't searched the Internet yet, that's worthy of a downvote. If the OP has searched, but isn't satisfied with what he or she has found, then that should be written in the post. "I've found Library X but I don't like it for reason Y."
Please help me understand the benefits of downvoting this particular question without providing a comment or context.
That's a fair question. Unfortunately, as of now the question hasn't received enough close votes yet to mark it as "on hold". Once it is on hold there will be a prominent message box detailing the reason and what can be done to improve the question.
There are so many bad questions posted that I presume many users are like me and let the close reason speak for them rather than commenting. That works well once enough close votes are received, but admittedly, it's probably quite mysterious and upsetting when the question is still open and the close votes aren't visible.