If you define a beginning/beginning relation between 2
tasks, it's not possible to define a gap between the
beginning of the 2 task.
e.g. "task 2 beginning = task 1 beginning +/- 1 week"
Unfortunatly, this gap could be very important to
create a gap when you want to organize the work of
your ressources, or when you are able to begin a work
when the predecessor task is not finished yet.