they are used as a control treatment in the example

they are used as a control treatment in the example. see explaination

DMSO has various role for which it is used like for freezing cells in liquid nitrogen, making solutions of various compounds in DMSO, etc.

here in the particular example shown in figure 7A. the compounds solutions were made in DMSO and thus to prove that the effect they are seeing is not because of the DMSO they have a set of sample treated with DMSO alone and other set with various compounds. After which they made the comparison between the two.

You can see in material and method section that the drug were prepared in 1% DMSO.