One of the most common grammar errors when writing is the contraction. Most grammar checkers can catch this mistake seeing how it is very easy for a writer to make that mistake. Essentially, a contraction is a shortcut. For example “it is” can be made into a contraction to read “it’s”. While you have other instances of contracted words such as “they are” / “they’re”, it’s is a lot more common because the spelling of the word with or without the comma is possible, whereas you can’t (can not) do that with theyre ( see?).
The mistake may come from the fact that many people may not know why contractions are used in the first place. This is one of those instances where an online grammar corrector may not know why either, but it knows when the use, or non use, of a contraction is wrong. This may not seem like a very big thing to some people, but when you make a living from writing and you’re (you are) in a position to influence people, every little detail such as this counts.
You will find a multitude of these such programs on the internet ranging from someones blog to University websites. Most of them are free, but there are some you can buy as well that may have more words referenced so, as a result, will be able to check more types of errors.
So whether you’re (you are) a casual writer who just wants to make a good impression, or a professional who needs to make a good impression, programs that check grammar are a very good thing.