What's the point of prepend communities?

[jason+nanog at lixfeld.ca (Jason Lixfeld)]

> On Oct 26, 2017, at 2:55 PM, Job Snijders <job at ntt.net> wrote:
> 
> If Network B offers some kind of â??Prepend to Network Câ?? BGP community, network A will be able to utilize all of network B except the pieces that perform less well. (This is ofcourse assuming that Network C picks some alternative path because of the prepends)

Absolutely.  I understand the "Prepend to Network blahâ?? use case.  The case I donâ??t get is where the ISP makes no distinction in their policy document about how the prepending of their own AS is applied to their upstream announcements, implying that itâ??s announced to everyone.