Punnett squares are used to find out the possible genotypes of the next generation after a cross and thus find the phenotype too according to the predicted genotypes.
For example, parents with genotypes of Rr and rr cross together, we can use the punnett square to list out the possible genotypes of their children which are Rr and rr.
You can never predict the exact genotypes of the next child but you can find out the possibility if they carry a certain trait. All the genotypes of the child are in random but you at least know what phenotypes they'll have.