Web vs. App

It is absolutely clear that there are many advantages in having offline access to your email, rss-reader and other stuff. But is it really necessary? I find myself downloading tools to access data offline a lot (setting up Mail.app or purchasing apps like Reeder.app for RSS). But in the end I tend to use the online counterpart (Gmail / GReader). I am sure that this is because I am mostly using my iMac which is stationary and always online. It surely is different while traveling where internet access is limited!