Why does the casino do that? Is it that they are scared of paying off that potential win if the game is finally successful? Or are they doing it to help the gambler take some profit so that even if the game bust, the gambler wouldn't be in total loss? The casino offers cash out when they are not even sure that the ticket will play out successfully, why?
The two answers you gave above might just be it, but for the former, I won't say that the casino is scared, they are not scared, but simply offering you (the gambler) and themself (the casino) the opportunity to save some cash.
This simply means that, if you the gambler accept the cash out offer, and the ticket generally end up not winning, you have saved yourself some money, that is, you gained your staked amount back plus maybe a small profit.
But if on the other hand, you accept the cash out offer and the ticket generally end of winning, the casino having paid you less than the amount in the tickets potential win, have saved themselves some money which they can possibly use for other expenditures.
I can't say or explain in full and understandable details why casino offer the cash out, but what I do know and believe is that, this feature contributes to making sports betting more interesting and exciting.
And point of correction, cash outs isn't only offered for parleys(multi-bet ticket) alone, single bets also get a cash out offer as well, though not every single bets.