La importancia de usar etiquetas (labels) en los formularios

El artículo Labels Required explica por qué es importante identificar los controles de los formularios:

Using the label element checks all of the boxes for inclusivity when properly formatted: it's directly associated with a form element (either with the for attribute or by nesting the element inside the label), displayed as text by standard browsers, and spoken by screen readers. Clicking or tapping on a label will focus the associated element (or select the checkbox or radio option), and when you focus directly on the element, screen readers will read the label. Ensuring that both visual and auditory cues are present and intelligible are critical when you consider that in WebAIM's annual screen reader study screen reader users report relying on a mix of both, with the majority using audio exclusively.

Hay otras alternativas a label, pero en este artículo se explica que no siempre funcionan:

It's possible to use markup other than label, but when you do, users may see or hear blank form elements and have no idea what to enter. 

At first glance, placeholder does a very similar job to label: it's displayed as text (within text inputs, specifically) and read aloud by screen readers. The placeholder has a purpose, though, that diverges from that of a label. It's meant to be a suggestion, formatting guideline, or hint that is ultimately replaced by user input. So while a placeholder may seem like a worthy label alternate on the first pass through a form, once a data value is entered, that label is gone. And that's just one of several reasons why placeholder is an inadequate substitute label. It's much more valuable when used as a guide. 

The aria-label attribute provides label text to screen readers for identifying interactive elements, or elements with ARIA roles, and it produces the same audio feedback as the label element. There's no reason to use both on the same form field, and we actually recommend against trying.

La conclusión es clara:

Out of the box, the label element is all we need to indentify fields in an accessible way. Used in combination with accessible hiding and other helper elements, like placeholder or icons, we can ensure that a form element's purpose is clearly communicated to all users.