Most SEO try to look for High PR Directories to submit their site links because of the following reasons.

1. High PR Directories can help boost your Google Relevance resulting to increase your own PR

2. High PR Directories are the ones frequently crawled by major search engines ending up as faster indexed of your backlinks in this directories.

3. High PR Directories are being prioritized by Google since they assume a list of relevant sites from these directories i.e. dmoz.org

4. Some SEO/Webmasters use such directories as their resources for link exchange partners that are related to their sites.