In this case a company in Kenya called Global Bio Energy is using discarded plastic products (bags, bottles, etc.) to make building posts. Global Bio isn't the only group doing this. See this You Tube video which does a great job explaining the process.
This obviously this helps deal with a tough trash problem and it also reduces the need to use lumber or other substitutes for building. Collecting the trash and turning it into posts will also employ local people -- though if these posts require less maintenance over time or are replaced less frequently the employment issue is muddier.
It would be interesting to know how this conversion process compares -- in terms of ecological & economic benefits or harms -- to harvesting lumber or making concrete or metal posts. While that's not clear to me what is clear is the never-ending inspiring ingenuity that entrepreneurs marshal to help solve some of our most serious problems.
(photo is from Green Options Wiki under "Cutting Down Plastic Bags Going To The Dump.)