Views are great for classifying content by type, author, date, vocabulary term, etc...
The existing "Taxonomy: Term" filter allow you to filter by one or more terms with a single operator at a time. Either all terms uses OR or AND but not both at the same time.
The Taxonomy Query Language fills that void.
The query language can combine query terms with different operators: AND, OR, XOR and NOT.
It complements the views user interface with filters for each vocabulary you've defined, allowing to only get nodes that match your criteria.