Why?: see the link I posted above.
That thread is talking about a missing label on a form. And the original respondent is actually incorrect. A title attribute would not satisfy the primary principles of WCAG 2.0 - that content is Perceivable, Operable, Usable and Robust.
Title attributes are not seen by sighted keyboard navigators (eg VR users) and may not be perceived by screen reader users either. So relying on title attributes fails Principle 1: Perceivable
Screen reader software in Forms Mode requires an association between form controls and their text labels - which is provided by the label attribute - not the title attribute. Without this association, the form may become unusable. So that fails Principles 2 & 3.
If you want to make your search form more accessible, add label markup and then position the label element off screen using CSS. Menu/navigation items don't need anything else - especially if you're using list markup.