I think tags give you option for another level of categorisation of information. Notebook stack is the direct form of categorisation of note inside them but tags inside each note create another level of categorisation which group the notes according to particular keyword. From the search point of view, Evernote can perform search in text inside note so search is no more a big criterion for justification of tags creation. Broadly tags rearrange the available notes in different architecture. Use the power of evernote to organise your office and home.