Over the course of long projects (and some short ones too), the shared understanding of the project, release, or even iteration goals can drift.  Different team members remember or interpret project aspects differently over time.  This drift can result in producing a final product that doesn't satisfy your client's expectations.  So how do we counter drift throughout the life of a project?  Some say "Write a vision statement and stick it on the wall?".  I'm not a big fan of "statements".  I think people get lost in them, and statements, over time, can be open to interpretation as well. 

Short of a unifying statement of sorts, how else can you keep your team synchronized with the goal's of the project, release or iteration?    That's where the beauty of agile practices comes in.  There are several practices which foster a cohesive, shared understanding of the goals: the iteration planning meetings, the iteration review, the retrospectives...but none more than the daily stand-up.  I think the daily stand-up helps anchor agile teams to their goals more than anything else.  On a daily basis, drift is kept in check by synchronizing the work underway.  If teams treat their daily stand-ups more as a synchronization conversation rather than a status report, I think the daily stand up can go a long way towards preventing drift from happening.