Reason for conversion of glucose to fructose in glycolysis
Avoiding diffusion is one reason to phosphorylate glucose, the other is that it is removed from the osmotic balance between inside and outside of the membrane, so it can be transported at a high rate.

The Glucose-6-phosphate can then be used as a substrate for different pathways, namely glycolysis and the pentose phosphate way, and (depending on the organism) also be converted into glycogen and starch for further storage.

The reason for the phosphorylation lies further downstream in glycolysis: The isomerization by the glucose phosphate isomerase and the subsequent second phosphorylation into Fructose-1,6-biphosphate make the conversion and dedication of the molecule into the glycolysis irreversible. 

Fructose-1,6-biphosphate is then cleaved by the aldolase into two C3-units: Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde 3-phosphate (GA-3-P). GA-3-P is converted into DHAP, so that downstream only one metabolite needs to be processed.