In this day and age, many submissions are handled through email. Agents and acquisition editors will often look at the submissions sent to them on an electronic device, commonly a computer or tablet screen. For many submissions, the initial contact is contained in the body of an email (no attachments). If additional materials are asked for, agents and editors expect things to be in the standard manuscript format. Yet, agents and editors will still look at those added materials using electronic devices.
So, if everything is now electronic, why must we format our manuscripts using a format that was devised back in the day when everything was printed? Well, believe it or not, the standard manuscript format is very specific for a reason.