In general, there are multiple possible reason.
- In some countries, immigration stamps the boarding pass to allow to later verify that you went through them.
- in some countries, security stamps the pass to allow later to verify that you went through them.
- you will get little sticky chits for your checked luggage, and they usually get stuck onto the boarding pass (but I have seen them stuck on my passport, which I hate)
- some companies require you to hand in the paper boarding pass as proof you really flew when filing your expense report
- their check-in process automatically generates the paper boarding pass, and they hand it to you.
Especially the last one is very common; I usually have an electronic boarding pass, and tell them I don't need a paper pass right in the beginning, and I see them print and discard it anyway.
You can simply toss it if you don't care to have it; but I recommend to keep it until you are on board, just to be sure.