Independent Bottlers (commonly referred to as IBs) are whisky companies that do no produce whisky themselves, but rather purchase whiskies from different producers and bottle it under their own brands. IBs play a large role in the whisky industry, and have since the very start, in some cases even predating the official producers (OBs) releasing their whiskies direct to drinkers.
The primary benefit of IBs is the variety they offer. Since they do not have standard releases, they don't have to always produce the same tasting whisky matured for the same duration in the same type of casks. Thus they offer an ever-changing selection of different ages, matured for different periods in different casks, from different whiskies producer, and often bottled as cask strength, single casks.