It is 100% necessary to include these in a voicebank, as I can see almost no one wanting to use a bank without them:
-A fully done oto.ini
-ALL the basic samples required for your voicebank style
-A readme so in UTAU I can hit their icon and read their readme, which generally at LEAST tells who made/recorded/designed the UTAU, release dates, contact information, terms of use (if you don't want your UTAU to be used in R18+ rated materials for example), and something that helps but I don't always see is best resampler/flags information and any character details.
Good to have, but not really needed however I like a bank more that has them:
-Icon art
-A sample section I can judge the voice by straight from UTAU (sample.wav)
-Extra samples (Like make sure you get ever form of every consonant)
-Extra language samples (Even just English Rs and Ls help, but don't go crazy with this)
-An omake folder that contains concept arts
Something definetly unneeded that just adds to the value of a bank:
-Extra supported styles
Like a CV bank recorded in CV VC JP style so it can also support CV VC, or vice versa; a VCV bank that can function as CV; I haven't seen it yet, but if someone someday makes an English CV VC bank that also supports JP CV, that would be amazing.
-More art than even the concept art, can be fun to look at if the creator is a great artist