One reason to do this would be to discover what other apps the user has on their device which may not be detectable by other methods. That is valuable business intelligence that could be used in various ways for maintaining a competitive advantage. I got this idea from this reply: https://twitter.com/nial_26/status/1167464788667928576
I would expect them to potentially grab the list of apps separately from that though (Some Ad SDKs do that, not sure if Facebook also does it - https://www.androidauthority.com/sdk-invasion-a-privacy-thre...)