What's the point of prepend communities?

In a message written on Mon, Oct 30, 2017 at 07:56:43PM +0100, Michael Hallgren wrote:

But keep in mind that 'prepend communities' are fragile: I decide by local preference whereto I send my traffic.

Absolutely, but they are still very useful in many situations.

Yes, sometimes very much so, and often nice to combine with local-pref settings based on upstream's "geo-community-values" when available.