I always thought that it makes better sense for websites to be centred horizontally to the browser window. But even an Adobe website is left aligned. Is this some reason behind this that I am missing?
For your average website? It's mostly aesthetics.
People have equated left aligned sites with a stable base, and center aligned sites as being flexible. I personally think it's all rubbish and that having your content in the middle makes sense, but that's my design preference.
In terms of actual usability, some people believe that left aligned sites mean the user will start reading the content quicker (or right aligned sites depending on the reading direction). At the same time, having your content centered means more equal white space which will give you better readability.
Finally, most quality websites are centered. Given that users expect good websites to be centered, it makes sense to center your own and not confuse their mental model.
As a side note, I think mihai is probably right with respect to Adobe, their alignment isn't consistent, but then again neither is the UI for their products so perhaps different teams worked on different parts /shrug.
I personally think left aligned sites look old-fashioned nowadays as more and more fixed width sites are being centred.
A lot of other Adobe product sites are centred - I suspect the newer pages are in the middle whilst the older ones are left-aligned.