[EvilFrog (OP)]

why glucose have to change to glucose-6-phosphate and fructose-1,6-bisphosphate during glycolysis?

[kilgorethecat]

Adenosine triphosphate (ATP) donates a phosphate group to the 6th and 1st carbons on glucose in a series of enzyme assisted reactions to prepare the molecule to be split in half, forming two smaller molecules each with their own phosphate group.  This phosphate group is essentially a way to store energy in bonds that will later be released and reused by adenosine diphosphate (ADP), and you will end up with a product of more net ATP molecules than what were used to fuel the reaction. 

[Yair Doza]

another reason is to prevent the glucose leaving the cell. Glucose enter the cell by diffusion through glucose channels. To maintain the gradient the glucose is modified with one organic phosphate - it can not go through the glucose channels any more. The second phosphate and modification are as explained so well in the previous comment.