Expanding your range of expression is often associated with obscure words often used in literary contexts. These words (e.g. supercilious, serendipity, etc) might be good to know, but as they are low frequency and low fertility, they will definitely not improve your fluency. Focus instead on high-frequency words used across a range of contexts.
The Word Bank section offers significant words from the world of business and finance. Under Foundation Business Words you will find a list of the Oxford Advanced Learners’ Dictionary list of 250 business and finance words. Each word has a definition from the dictionary followed by a translation used in this context. It also has samples of usage.
Other sections in the Word Bank focus on eliminating common translation problems and expanding your range of expression in addition to pronouncing words correctly.