The edge has 2 primary purposes: to reduce variance when you are deep stacked and to help account for future hands when you are very short stacked.

When you are deep stacked you may not want to push very small Diff%s because it will increase your variance.

When you are very short stacked you may want to push small negative Diff%s because you probably will not have a better opportunity in the future.

The default edge is calculated to take these these two factors into account. This FAQexplains exactly how the default edge is calculated.

You can disable the default edge calculation by selecting the Zero edge model as the default edge model in the program options.