It is nakedly a psychological trick to give you a mixture of things you've seen before and new content to make you feel like you should try again more often - the slot machine strategy. No one asked for it.
Of course it helps that ads are easier to integrate into a feed when you don't even know what to expect in your feed in the first place.
So now, not only do these platforms not give a feed of pages and people I subscribe to, they litter my feed with ads and 'popular content' that I never subscribed to! otherwise how could they charge money to increase your organic reach?
what a crock! It was a marketers' dream to have their latest news and coupons delivered to your inbox and have you actually look at them, but if you subscribe to businesses you want to follow, how can the ad network make money? So they turned everyone into a customer of their ad platform, and held your content hostage until you paid up. This has effected numerous friends that are small content producers that maxxed out facebook's 5,000 friend limit and all they wanted was to invite all of their fans to their next event.
/rant