Other than the details of your essay, grammar is the hardest part of your essay to do correctly. This is because the way we talk and the way we are supposed to talk by English standards is very different. Every area has a different way of pronouncing words and putting sentences together. This makes it harder to write in a grammatically correct way, because very few people speak the way we are supposed to write.
It often seems like it is only the very rich and posh who talk in the same way we are supposed to write. People, who write custom papers for a living, are trained in this matter. So they do not need to worry about getting the grammar right, because they write custom essay all the time. If you are writing custom term papers for a piece of custom coursework at home, then you can use the spell checker to catch any spelling mistakes, but it is pretty useless at catching grammatical mistakes. This is because we can write plenty of stuff that makes no sense, but is grammatically correct nonsense. If however, we were trying to make a point, then the grammar would need to be geared towards that point. It is very difficult for a computer to figure this out.
Your computer is not going to pick up on a typo. People who write custom papers are paid to check for every possible error they can, but a computer does not know better. There are plenty of typos that are both spelt correctly and grammatically correct. How many times have you written a typo such as "top" instead of "to" because your finger accidentally nipped the "p" just after you typed the "o"? Top is a word that is spelt correctly, and in most cases it is going to be grammatically correct because it can be stated in many different ways. A person can blow his top, or be the top man. You can reach the top of a mountain or be in the top ten. A computer program is not going to be able to pick up on any grammatical errors around the word "top", and that is just the top of the iceberg, (please note how easy it is to type "top" instead of "tip").
There are literally hundreds of typos that you could accidentally make, and your computer is not going to pick up on them, and because they look so similar to the word you were expecting, you are going to have trouble picking out the typos from your custom paper. Omitted letters are similar typos that are very hard to catch when reading through your custom essays. Accidentally not typing a letter can lead to other words that are so similar to the intended word, that most people do not notice them. Such as when "your" turns to "you" or "they" turns to "the". Your grammar checker may pick out one or two, but not all of them. A custom writing team are trained to do it.