Ask HN: What makes you switch from desktop to web application?

Recently I have seen very complex "wedding excel", large spreadsheet file with many sheets dedicated to tracking tasks and information related to wedding preparation. As usual, excel file was shared via email and versioned manually ... :)

There was no special feature which required desktop version - it would be piece of cake to accomplish same task using web alternative (for example Google Docs). And a lot faster.

My question is: what makes you switch from desktop to web application?

Compared to desktop most web apps provide less features, but do not require installation, provide new features like "use everywhere", cloud storage and sometimes Ehterpad-like synchronization. However with services like DropBox cloud storage is not that big advantage any more.

So what was the killer feature which recently convinced you to abandon desktop <insert your favourite application here> and start using web app?