You may not know what to call them – balls, fireballs, numbers, etc. – but there’s a slot industry term for those symbols with numbers on them, usually connected to some sort of mechanic like Hold & Spin. The term is Cash on Reels.
The concept was popularized by the Lightning Link series of games, but has gone in all sorts of different directions since, with various game types coming out with ways to collect the cash found on the reels.
Konami has taken all sorts of swings at Cash on Reels games; Thunder Arrow and Money Galaxy are a couple that I have had fun playing in the past. Everi has been tinkering with different ideas too, on games such as Cashnado and Gold Hills. Bally has Ultimate Fire Link, Jinse Dao, Monopoly Money Grab and Cash Falls among its titles with the feature.
Of course, one of the downsides to the format is the cash on reels can many times act as a blocker of other winning combinations by getting in the way (unless the feature that collects them triggers, for instance). And the cash on reels feature will take some pays out of the base game
So why are we seeing so many of these sorts of games? Because people are playing them quite heavily, and manufacturers are out to give players what they want! The reality is these sorts of games tend to get heavy play when they catch on, because the cash on reels element tends to be in addition to some other bonus or feature, which means there’s more to play for. And for many players, that’s what they want right now.