Centrifugation involves separating particles of different sizes, masses, density and etc.
In the experiment, the DNA macromolecules are suspended in a solution of cesium chloride gradient and then centrifuged.
Why is cesium chloride needed? Considering the possible DNA produced in the experiment will be the same size but of different mass resulting in different density, DNA will be separated based on their density by virtue (without the need for cesium chloride) under the influence of a centrifugal force.
I noticed someone asked the same question on Khanacademy but there was no answer for it. Other websites I found doesn't seem to offer a valid answer e.g. here which says " the DNA comes to equilibrium in the gradient where its density equals the density of the surrounding CsCl" which I think doesn't really matter when considering the argument I proposed above in the third paragraph.