WITHOUT_ARRAY_WRAPPER option enables you to generate a single object instead of the array. Use this option if you know that you will return single row/object:
SELECT top 3 object_id, name, type, principal_id
FROM sys.objects
WHERE object_id = 3
FOR JSON PATH, WITHOUT_ARRAY_WRAPPER
Single object will be returned in this case:
{"object_id":3,"name":"sysrscols","type":"S "}