Visually, a label placed next to an input field looks associated to a sighted user, but without a real programmatic connection (a label element with a matching for attribute, or the input nested inside the label), a screen reader has no reliable way to connect the two and may announce the field with no name at all.
Placeholder text inside an input is not a substitute for a real label, since it disappears the moment the user starts typing and is often skipped or mishandled by assistive technology and does not meet WCAG requirements on its own.