What's the point of prepend communities?

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

Hi,

Of all the ISPs that I am familiar with that have a BGP community structure usable by their peering partners and/or downstream customers, among other things, they allow the customer to signal the ISP to prepend their own AS to the as-path of a particular prefix announcement.

What functionality does a provider prepend support that is otherwise lost in the absence of such a feature, but all the while, the customer would be able to prepend their own AS to the same prefix announcement anyway?

Is this a relic from before ISPs allowed for local preference adjustment, or is there actually a use case for this?

Thanks!