Browsers will often offer to save passwords, and autofill them into forms. Since most people don't protect their autofill with a password (or leave autofill unlocked), and additionally leave their computers unlocked (or even password-less), this exposes many passwords to an unsophisticated physical access attack in which the attacker copies the password out of the form field and into a non-obscured text field.
Having to inspect the page and dig through the html to find a password isn't particularly difficult for someone with some technical knowledge, but it narrows the field quite a bit from people who just know how to copy and paste. This makes it a small-downside wide-impact security filter.