As most of you know justifying type means that the margins are even on both the left and right margins. It’s a way of organizing lots of text and making it easy for the eye to read.
Unjustified type sets the type ragged on one side. This can also make for an attractive and readable setting.
In order to justify type, space is inserted between words and, sometimes, letters, so that things line up. To a certain extent, this additional space compromises the integrity of the type and makes it less attractive. However, the overall effect can be that the eye has an easier time reading.