The thing is, vu ヴ, as it's pronounced, is a katakana used primarily for foreign words starting with the v sound in Japanese. This is why a hiragana version of the ヴ.katakana is nowhere to be found, because foreign words are often written in katakana, not hiragana, (see Japanese alphabets)
Japanese people can't speak English, so the vu ヴ kana is a practical necessity to emulate any words that start with v. Remember: there are no vavivuvevo syllables in the Japanese alphabet, so there are no native Japanese words that start with V, just like none of them start with L.
So when a word starts with V, like "virus," it can't be Japanese, but it can be imported to Japanese. When it's imported it needs to be written in katakana, so it becomes... huh... bairasu バイラス? No? Not good? Then uirusu? ウィルス? Not good either? How about vuairasu ヴァイラス then? Still no?
Welp, then it's no then. No matter how you do it (even with the vu ヴ) you can't actually say the word "virus" in Japanese the way it's said in English. The same goes for words like "violin," vuaiorin ヴァイオリン and "vitamin," vuitamin ヴィタミン.
Why It Exists Then?
It's obviously to approach the foreign pronunciation better. Japanese already has plenty of trouble with "la" becoming ra ラ, using the combination of vu ヴ plus one small-kana (ァィぇ) to make sounds more similar to va ヴァ, vo ヴォ, ve ヴェ, etc. helps clear up some confusion, which is good.
For example, the word "best" is imported as besuto ベスト in Japanese. If you were to say "vest," however, it would end up with the same word: besuto ベスト. Imagine asking someone for their "vest" and they think you are saying "give me your best." That's just plain confusing. With vu ヴ, you get a word like vuesuto ヴェスト, which is slightly more accurate.