Assume we have a fruits table. And we want to select all the fruits that have a red or orange or blue color. But we want the resultset to have first all the fruits with color=red, then the fruits with color=blue and then the fruits with color=orange. It is obvious we can not use an alphabetical order. Thus we have to introduce our own order relation.
select * from fruits where color = 'red' or color = 'blue' or color = 'orange' order by case when color = 'red' then 0 when color = 'blue' then 1 when color = 'orange' then 2 end