Specification and Use of Fiber Voids

[DaveVikingPE]

I've been asked about fiber voids in concrete construction, namely slabs and wall. My understanding is that they're placed in concrete structural elements that are exposed to the elements where the fiberous material can degrade over time. The use of such would be for a deliberate void in the structure. There's not much on the web (found a company that makes 'em, but not much more literature; the USACE guide specifications calls them out, too). Why would I want to use fiber voids for anything larger than a weep hole? Why wouldn't I simply embed a pipe, etc.

Thanks in advance for any info!