We noticed a while ago that, if you look at the PageRank of a page (how reputable we think a particular page or site is) the ability to spell correlates relatively well with that. So, the reputable sites tend to spell better and the sites that are lower PageRank, or very low PageRank, tend not to spell as well.
Although we do not know for sure if grammar and spelling are a direct ranking factor for SEO, it is clear that it is crucial for the user experience, which with the introduction of artificial intelligence machines like RankBrain, is gaining a lot of weight among the most important ranking factors.
Plus, when considering the difficulty and the time dedicated to other important factors likes backlinks or content creating, the extra 5, 10, or 20 minutes to check your grammar and spelling are a bargain! I know I sound like a broken record, but grammar and spelling are important!