Depends on the model
Older models I have used on the road, or away from office/home typing, I love the 2008 keyboard for some reason.
I have previously used old models for the same purposes as today, but the same software and applications tend to pull more now than they used to.
Ever since moving onto i5
Photoshop for image editing, cropping but I have been finding myself using the Mac photo editor more and more. So intuitive, and I only went with photoshop because of using it on desktop, but for basic stuff and overlays, the editor works great.
Sony Vegas for video editing
Traktor, Maschine, Ableton for sound and music production dabbling, more of a hobby, but instead of building yet another Digital Audio Workstation again, I just dock my mb and this way I can still take my stuff to the music studio with friends who do this for a living, rarely happens but its' fun to have the versatile power and mobility.
I love the macbook because it often runs with my motu soundcard much better, latency is always low, its' hard to get the same level of performance from a PC.
I rarely use them to browse because I tend to do the millions tabs on my desktop with plenty of ram.
(never splurged for i7's due to various throttling issues)