In my experience the reason for a JSON object per line is because a tool can then split the entire file by newlines and have a list of objects to start parsing/

In my experience the reason for a JSON object per line is because a tool can then split the entire file by newlines and have a list of objects to start parsing/processing in parallel, which avoids having to parse the entire file up front to get usable data, and lets the tool start processing things while some data is still being parsed.