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Radio is facing competition from streaming without those other revenues, so CBS thinks shareholders will be better off without that division, using the cash received to invest in films, productions, and platforms which will garner higher revenues. CBS is in "invest" mode, iHeart is in "harvest" ["milk assets"] mode and will try to use the businesses' own cash flows to pay for it, therefore they need much higher profitability than CBS does out of the same stations. Very different business strategies.
Actually, CBS did not "sell" the radio division, and shareholders are not "better off without radio" since those same CBS shareholders now own, separately, shares of Entercom... 72% of the entire Entercom pie, in fact.
CBS corporate is divesting itself of the radio division. They don't really get the cash value but, instead, pass it on to the shareholders.
There is nothing to put in the CBS corporate kitty to invest in other films, productions and such. The do, however, get about a thousand million dollars of debt transferred over to the enlarged Entercom as part of the package.