In order to use single quotes, like in this example, you have to concatenate in your variables. You don't have to do that if you use double quotes. However, it can be harder to see if you've gone wrong if you always use double quotes.
One can use either single or double quotes to get just about anything done. It is just important to know how to use single or double quotes correctly. I have heard people say that they only use single quotes and prefer to concatenate all variables in. This also makes most sense to me as you can easily see if a mistake has been made and where that mistake is.