I like the possibility of being able to automatically post my Etsy listings on my Facebook page, so even when I'm busy and I'm not at the computer Seller Tools does the job for me. I have increased a lot my shop views and sales thanks to Seller Tools!
I especially like the promotion tools which has easy schedule set up with all options I need. Saving a LOT of time on a daily basis! Keyword and trend tools give information to think about and to improve the tags and titles, really helpful. And since this is dynamic, you can stay tuned to the changes in the area of your interest with no waste of time for search it in other way.