The person you choose to look over your statement should be checking everything. Along with grammar and spelling errors they should check the tone of the paper, the flow of the paper, and the overall style of the paper. So, it makes sense that the person you choose to look over your admission essay should be well tuned in writing. Additionally, they should also be able to let you know if there is anything they feel you should omit or add. Once you have made the changes they recommend, then you should have them re_read your personal statement to ensure quality. If you want to be ultra_safe, then have yet another set of fresh eyeballs look over your revised personal statement.
As mentioned, this part of your application can be used to decide whether a school wants to invite you for an interview or not. Schools only accept students who have been invited for interviews, so getting to this step is a huge part of the process.