In glycolysis, the isomerization of glucose to fructose is necessary because

"it is needed to ensure that equal 3-carbon units can be made."

One important part of the glycolysis is the metabolization of two three-carbon molecules. A glucose molecule could not be divided into two molecules. 
After a phosphorylation of the fructose, the molecule has now two phosphate groups. The molecule is then divided into two and the glyceraldehyde-3-phosphate (one of two) continues the chain reactions of glycolysis.