Adding Time brings the two into a comparison of time periods. Saying just Day and Night compares the non-time/instantaneous status; light and dark, up and down, inside and outside. This is the purpose of the usage, not a time period.
The expression by night is typically used to contrast someone's nighttime activities to their daytime activities, especially when the nighttime activities are unusual or unexpected. You will see it very frequently in the construction X by day, Y by night. Some examples: ...treated it like a greasy spoon by day and a hotel by night I mean, if it's -- if it's somebody that is -- is living as a ...
- The origin of "at night" to indicate a point of time and the usage of prepositions "in" and"at" In olden times, when the time expression "at night" was originated, night might have been thought as a point of time in the day because there wasn't any activity going on and people were sleeping that time unlike daytime.
At Night or In the Night? - English Language & Usage Stack Exchange
By saying "have a good night", you're wishing them well for the entire remainder of the day, which is more informal than simply saying "see you tomorrow". An alternative would be "have a good evening".
politeness - Should I say "have a good night" at 5:00 PM? - English ...
Can we use "day and night time" instead of "day and night"?
What can I say about a thing happened at night? Someone stole my phone at night. OR Someone stole my phone in the night. Which one is right to say?