1. Our cat is cuter than those in the shelter
In the first sentence, for 'those' to have a clear referent we need a plural noun. Notice that 'cat' is singular. Therefore 'those' could not be referring to 'cat.'
2. Some people believe that the benefits of a healthy diet outweigh those of regular exercise
In the second sentence, 'those' refers to 'benefits,' which is plural.
Hope that helps
Magoosh Test Prep