When someone follows you on Twitter, do you go and look at their profile to see what their talking about?  Do you scan their profile page to see what type of conversations they partake in, if their a perfect fit for your brand?  If you don't, it may be a small practice that you need to partake in.  If you do, then you'll totally understand where I'm about to go with this next sentence.  

Pinned Tweets tell interested people how you communicate and what your brand is all about.

When you're not talking about current industry conversations, there is no better place you can use -  to post free product information, free download offers or even free courses that you offer.  

KarmaO Tip:  
You can use a pinned post to introduce new clients and industry peers about  a blog post that you know others need to read about.