OK, imagine you work at a big company. Like any big company, there's a push to make the quarter's numbers, to grow the customer base, to retain existing customers, to increase market segment share, etc.
Many web services make it difficult to leave their services - you have to pay them for exporting your data, or jump through all sorts of technical hoops -- for example, exporting your photos one by one, versus all at once. We believe that users - not products - own their data, and should be able to quickly and easily take that data out of any product without a hassle. We'd rather have loyal users who use Google products because they're innovative - not because they lock users in. You can think of this as a long-term strategy to retain loyal users, rather than the short-term strategy of making it hard for people to leave.
We've already liberated over half of all Google products, from our popular blogging platform Blogger, to our email service Gmail, and Google developer tools including App Engine. In the upcoming months, we also plan to liberate Google Sites and Google Docs (batch-export).