The katakana syllabary is used primarily for transcription of foreign language words into Japanese and the writing of gairaigo (loan words), as well as to represent onomatopoeias, technical and scientific terms, and the names of plants, animals, and minerals. It is also occasionally used in some words for emphasis, or to ease reading; katakana may be preferred for words becoming buried in the text if they are written under their canonical form in hiragana. Names of Japanese companies, as well as certain Japanese language words such as colloquial terms, are also sometimes written in katakana rather than the other systems. Formerly, female given names were often written in katakana. 
This is a relatively recent addition to kana usage to represent the non-native sound vu. As such, this is only found in borrowed words. Many speakers, especially older people, may pronounce this more as bu, [bɯ̟ᵝ]. As a result, it may be replaced with ブ(bu) with the same pronunciation. One example is ヴードゥー(vūdū, “voodoo”), often replaced with ブードゥー(būdū) or ブーヅー(būzū).