皆さま おはいようございます！
I have been using japanese keyboard on my phone for quite a while. (Most if the times, to check my reading before i submit my anwser in WK. はは)
I have notice some weird “selection” for my input hiragana like ﾀﾝﾀﾝ . And why the needs of 2 types of romaji; tantan and ｔａｎｔａｎ.
My question is, why and when I need this type of selection? 
They existed primarily for monospaced font display in old computing contexts. But now half-width kana generally shows up only as an aesthetic thing or in very specific settings where space must be saved.
Full-width Latin characters show up plenty in typed Japanese text, it doesn’t mess up the monospaced alignment and I suppose the Latin text stands out a bit more. But you’ll also see it written ‘normally’ plenty.
As for typing it yourself, maybe on some site registration forms…? I can’t remember ever having to use half-width, outside of Japan… Just use full-width hiragana and katakana like normal. (タンタン is katakana btw)
Japanese text input is a mess, sometimes you need to use a certain style to be accepted. For example when Japanese web forms want Kanji input it also accepts full-width romaji but nothing else.
I mainly see half-width kana when dealing with banks, I guess their computer systems are still prehistoric and they don’t want to touch them.
I often have to use full-with romaji when I enter my name in Japanese forms that require separate fields for name and furigana. Generally Japanese people just use kanji, hiragana, or katakana in the name field, and those are all full width. They don’t expect half width romaji I guess.
Thanks for the reply and the link!
Yes, katakana. 
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