Mobiles (and tablets) are supposed to be more than just for data consumption. You should be able to get work done on them. While I do not see myself making a sales deck on my smartphone or writing 5 level nested formulae on an excel sheet with my tablet, I definitely need to read, create and reply to emails on them and keep track of meetings and tasks.
While apps for social networking (facebook, Twitter, Tweetcaster) and browsing (Chrome. What else?) are well entrenched in their popularity, emailing apps on the phone are still something we use only grudgingly. Its a necessary evil. And when it comes to task management there are even fewer people who use any app – most I know prefer writing it on a white board or Post-its or making an excel sheet (the MBA answer to all problems in life).
Like most of us, I need to be able to access my emails from anywhere. I also like to check my tasks for the day first thing in the morning. I also attend multiple calls a day and like to take notes for reference in future and tag them for easy recall. These are the 3 apps that I use for these tasks and I absolutely cannot live without them