Quick Bar displays Promoted Tweets and trending topics. According to Twitter:
The QuickBar was originally conceived to help users discover what’s happening in the broader world beyond people they already follow. The bar was also seen as a potential means of in-app notifications for new mentions, DMs, and other important activity.
New Look, No QuickBar
We believe there are still significant benefits to increasing awareness of what’s happening outside the home timeline. Evidence of the incredibly high usage metrics for the QuickBar support this. For now, we’re going back to the drawing board to explore the best possible experience for in-app notification and discovery.
If there’s an incredibly high usage metrics for the QuickBar, why removed it? Anyway, I’m for the removal, as QuickBar was an unnecessary distraction to mobile tweeting experience.