“By far” is commonly used with superlatives to emphasise the difference between different things and can also be used in other contexts to show a great amount. Candidates can use this phrase in the speaking or writing papers when making comparisons, and they will also can be tested on this in the reading and use of English papers, particularly parts 2 and 4.
Examiner: “What is your favourite thing to do on a Friday evening?”
Candidate: “My favourite thing to do by far is just stay at home on the sofa with a pizza and watch a film…”
Seeing all the different animals in their natural habitat was by (4)…… the best thing about the entire trip.