what is the purpose of a bearing spacer?

Make the length of the spacer the same as the distance between the sholders in the hub and you will be good to go.

this is the story of the three bears and a front wheel spacer.

the one bear said if the spacer is to long it will bind.

the second bear said if the spacer is to short it will bind.

the third bear said if the spacer is just right, all is good.