An array should not be used similar to a relation. It should rather contain indexed values that relate to one row very tightly. For example if you had a table with the results of a football match, than you would not need to do
id team1 team2 goals1 goals2
but would do
id team[2] goals[2]
Because in this example, most would also consider normalizing this into two tables would be silly.
So all in all I would use it in cases where you are not interested in making relations and where you else would add fields like field1 field2 field3.