Some small Katakana use "x" followed by "i", "u", "e", etc. which makes characters such as "ィ", "ゥ", "ェ".
Typing "xx" will make "ッx", and if you follow that up with "i", "u", "e", etc., you get "ッィ", "ッゥ", "ッェ".
But if you want to type something like "メッセージを入力", how can you type "ッ" without deleting the leftover "x"?
I should mention that this is on a Mac, but I see that this holds true on Android's Google Japanese keyboard as well.