Sunlight makes it easy to see changes to crawl rates by pages and sections. It's a warning sign that they might not be linked into the site well enough, or that they might benefit from link building.
Find and fix duplicate content issues, and see how frequently Googlebot renders non-SSL pages, noindexed pages or pages that canonical elsewhere. Improve how Googlebot crawls your site, and you can improve how well your site can rank.
Search Console can tell you which issues Googlebot has found - but Sunlight can tell you how often it renders them, so you can build a stronger case for fixing broken pages and start seeing real improvements.
Slow loading pages have lower conversion rates, and can rank lower in search. Sunlight automatically measures your page load times, so you can find and fix performance issues faster than ever.