The value attribute can be used as a default placeholder or a preset value for the field. When the field is selected the default value is then highlighted and can be changed by the individual. Other than that, the "value" attribute is not required to process the form, the input entered will still be submitted.
HTML:
<input name="favoriteCities[]" value="" />
<input name="firstname" value="Enter Your Name" />
<input name="firstname" />
Note: It was used as a placeholder for many years until the created the new "placeholder"
attribute for some reason, kicks and giggles I guess.