This is mainly for people that live in or have lived in Japan but others are welcome.
In what instances have you seen the word ジャパン being used instead of 日本? Is there any specific reasoning or pattern that you have noticed or been told of. I guess this question can be expanded any common loanwords that you see people using.
Of the top of my head, I can imagine people using オ-ルジャパン instead of 全日本 or 全国. It makes it sound “cooler”?
Some names of places, shops etc. that have the word “Japan” in them are just put into katakana. One of the instances I think of is ユニバーサル・スタジオ・ジャパン (Universal Studios Japan).
In anime / manga / video games / etc., it’s often used by characters that are foreigners, in order to represent how they say “Japan” in their native language (which is usually English) or to help give the impression that they have a foreign accent.