When hiring someone - be a company or an attorney, for example. Their use of language is a reflection of their power, control and commitment. Can you imagine someone in customer services, smacking gum, in a sparkling boob exposing cheerleader shirt, saying 'like' 'like' like', and other high school jargon to a nice old Polish lady about a $500.00 billing error? How about a neurosurgeon, with pimples, filthy hair, piercings, political and religious tattoos, using teen age terms to describe clipping that aneurysm about to explode and kill you? (Ok, maybe I'd like this doctor, to talk to in a bar, but I would be thinking: "Get your dirty knife away from my head!!!") Words and grammar are verbal hygiene. Words say who you are and what you care about - in a micro-second.
You are going to need either one of two skills, if not both, to secure a decent salaried job: Proper grammatical and spelling skills or college-level mathematical abilities. Grammar is especially important on a resume, to show that you are educated and capable to work on your own without any assistance.
While it depends on the job, good communications skills tend to be very high on most employer's priority list. I have helped with hiring for an organization, and if we saw a typo or grammatical error on a resume, we would toss it out. Grammatical errors show a lack of thoroughness.
No one would want to hire anyone that had sloppy grammar on either an application or interview. They would want someone that knows English well to work for their company. If you do not have proper grammar it shows a lack of education and knowledge not exactly the best employee material.
When in a work environment, respect and knowledge are very important and someone who sounds like they are educated, through proper grammar you will earn respect and eventually more money. Even the smallest mistakes can leave a lasting impression on a future employer or a current boss, and clarity of your speech and righting leaves a good impression.
Yes, knowing how to use -- not to mention actually using -- proper grammar will indeed help anyone get a better job. We are world that relies on the written word now more than ever -- emails and texts are king. If you can't construct a proper sentence, you'll get looked over in the workplace.