|This page in a nutshell: When an article contains unverifiable content or lacks vital content, it needs to be corrected now before someone reads it and is misled by it.|
Wikipedia is one of the first sources many people check when doing research. As a result, any misinformation found here could quickly spread, and should be immediately corrected before any damage is done. As a corollary, when an article lacks vital content, that content should be added as quickly as possible.
Google any word, and there is a good chance a Wikipedia article will be the first or second search result. Moreover, many of the results lower down the rankings are likely to be sites that mirror Wikipedia. Wikipedia is unavoidable.
For this reason Wikipedia is frequently the first thing people read when, for example, they wish to find out about a political party or candidate during an election. Although it ought not be the final stop for someone seeking information of this kind, its ease of access frequently does make it the first and last source of information for many people.
Some people will tell you there is no deadline, because all errors will be corrected in the long run. That may or may not be true, but most people won't keep revisiting an article every week as it gradually improves. They will read only one version of an article: the one that is available right now. Therefore if an article contains false or unverifiable content, please correct it yourself. Immediately.
Misinformation can trickle from a Wikipedia article to a published secondary source. If that source matches Wikipedia's guideline on selecting reliable sources, the misinformed source can now be used as a citation to back up the inaccurate Wikipedia article, and to give it still more false credibility. A vicious circle emerges: false information in Wikipedia article is included in a published source, that source is cited in a journal article, and then that journal article is cited again on Wikipedia.
Wikipedia's focus on 'verifiability over truth' means that Wikipedia policies are now powerless to discredit this untrue but apparently well-sourced claim. Wikipedia policy notes this danger in WP:CIRCULAR. This way falsehoods grow on Wikipedia like weeds, and it requires a great effort with many reliable (really!) sources to uproot them.
In 2012, the authors of the Leveson report were taken in by a Wikipedia editor who named a fellow student as a co-founder of The Independent newspaper. The Leveson report easily met Wikipedia's criteria as a reliable source, and would likely have been used to support the original claim had the prank not been discovered. Fortunately this error was corrected, but how many times have similar mistakes happened and never been detected? Rather than let this happen, it is far better to remove manifestly false content from an article, now.
We can disagree over whether this or that article is about something important. Importance is highly subjective. But whatever your views, a great many Wikipedia articles are about something important to you. There are articles about everything under the sun, the sun itself, and everything beyond it.
If an article has been written at all, then its subject is important to somebody. If an article survives deletion proposals, or is never nominated for deletion, then it satisfies Wikipedia's notability criteria, and its subject is important enough to deserve accurate treatment.
Wikipedia has a massive effect on what people think: there wouldn't be any point to Wikipedia if it didn't. When a corporation uses Wikipedia to unfairly disparage a competitor, or a government to smear an enemy, it will succeed for as long as it remains unchallenged.