You tend not to adjust cellspacing a lot. Editors typically use a default cellspacing value of 2-3 pixels. Authors probably change this most often to
- put some space between elements in a table
- eliminate different-colored gaps between cells in tables whose background colors contrast with the page background color.
An example for (2) above should make (1) clear as well.
Examples:
<table border="0" cellspacing="5">
| ------------------------ | ------------------------ |
| ------------------------ | ------------------------ |
<table border="0" cellspacing="0">
| ------------------------ | ------------------------ |
| ------------------------ | ------------------------ |