For making a sale, you need to have a willing buyer. If you want to sell it in one transaction, you will need to do it in a reliable exchange that has a good volume and can handle that transaction. Since 100 BTC is a big number, a lot of these places will need you to have passed certain verification levels which is not something you can get done right away.
It makes sense to break that 100 BTC into smaller blocks so you can get your transactions done without the risk of having it raise concern. Also, you don't want to make a mistake which can render your transaction stuck in the blockchain.
But why are you in a hurry to sell 100 BTC in the first place? ;-)