SteemPeak has a lot of really good features you just don't get with the Steemit front end.

Beneficiaries, is one, for sure. The ability to save drafts and create templates for basic info you might use in multiple posts (footers, gaming templates, etc.) is also very useful — often have an idea but only enough time for a quick note, not for the finished post.

Steempeak is one of the very few places you can bring up the entire list of witnesses, not just the top 100.

Used with the "drafts" feature, you can also schedule posts... great if you're heading out for a weekend but still want to post something every day.