what is the purpose of fixing a slide that is to be stained

 

In the fields of histology, pathology, and cell biology, fixation is a critical step in the preparation of histological sections by which biological tissues are preserved from decay, thereby preventing autolysis or putrefaction. The structure of a tissue is determined by the shapes and sizes of macromolecules in and around cells. the principal macromolecules inside a cell are proteins and nucleic acids. Fixation terminates any ongoing biochemical reactions, and may also increase the mechanical strength or stability of the treated tissues. The broad objective of tissue fixation is to preserve cells and tissue components and to do this in such a way as to allow for the preparation of thin, stained sections.

 

http://en.wikipedia.org/wiki/Fixation_%28histology%29

Also note that not all imaging techniques require fixation and in some cases it can be detrimental. Though stains tend to be invasive enough that it does not matter much.