attack on titan before the fall is 70 years before Erens time its also that time when humans still didn't know how to kill titans properly and before they even invented Maneuver Gear.
Doesn't it seem interesting,how did human kind invent the Maneuver gear?
How were things before the original Attack on titan?
Well then this manga shows you the past which is very interesting.
This manga has nothing to do with Eren or it seems that nothing links,its a story about different people and their problems the only similarity is that they live in the same walls and have titans surrounding the walls and that human hate
the titan which didn't change.
Reading another in another perspective view of a different person is something that is useful.
Attack on titan before fall has romance in it which is very different from the original attack on titan which makes the before the fall more unique.
Overall I think this manga is Interesting and it reveals the past.