Here's what we use them for in our own SEO campaigns: 

• T2 link building - (sending more power to your Tier 1 links)
• Tier 1 link indexation (speeding up/helping with the indexing of your Tier 1 links)
• Anchor Text Diversification (diversifying your link profile with low risk links like generic links, naked links, non-keyword related phrases, etc. )
• Testing new SEO strategies (incredible powerful way to safely test new SEO tactics)
• Churn & Burn SEO projects