when you want distinct values from one column then you have to decide which row you want when there are multiple occurrences of that value from that column.
As an example:
City, time, temp
Niagara, 2 p.m., 75
Niagara, 3 p.m., 80
Buffalo, 2 p.m., 81
Chicago, 1 p.m., 92
which of those two rows do you want for Niagara?
DISTINCT is not a function, it works across all columns from your table, not on a single column. So all rows in the example are DISTINCT from one another.