"Banks close at 5."
Well in France unless you are at an important city you should finish any kind of shopping before 6 pm. Kind of annoying if you have to travel through France by car (also small gas stations work mostly by credit card, so gl interests if you arent from France...) Yeah, nothing to do with Japanese, just a random fact related to that time
Yes, that's one of its usages. When saying something happens at an exact time に is the equivalent to "at". "At 5 o'clock" = 五時に.
In this case に acts a specific time marker so it's specifying that banks close right at 5.
You need ni after times and specific days, so you need it to say "at five".