The desktop Office apps have been connecting to Office servers for years now, from the original Project Server that grew into SharePoint to automation that lets a business programmatically create documents. (At the risk of igniting an old argument, the ability to construct documents automatically using code is one of the main reasons for the Office XML file formats — you can't do that with a binary file format, but once your file format is XML, you can slice and dice, poke, prod and rebuild documents wholesale without ever firing up a ribbon-based application.)
One of the most popular Office Server tools is Excel Services. At one level, this avoids the multiplication of spreadsheets, where everyone take a copy, makes a couple of improvements and uses it to run their department; come the end of the year, someone has to make all the figures add up from multiple spreadsheets with multiple and subtly different dependencies. It's also part of making business intelligence and number-crunching more accessible. Keep the data in the database where it belongs, put the spreadsheet in SharePoint, serve out the calculations through Excel Services, lock down the parts that need to say the same, and everyone can work out what they need without changing the assumptions on which your budget is based.
The Excel Web App embed option
Excel Services in action on a Web page
Also important is that you're doing it without ever going near SharePoint — although as the same engine drives the Office Web apps on SkyDrive as on SharePoint, so this raises some interesting questions about what you'll be able to do with SharePoint Online going forward. As of the first Service Update this autumn, SharePoint Online supports the Excel Services REST API, and a version of Business Connectivity Services for connecting external data sources. At this point it only works with Windows Communication Foundation services and you only get read-only access to external data, but given how recently Office 365 launched, that's a reasonable start.
The SkyDrive team claims that the site is "already getting 50% of the Google Docs page views"; along with the new SkyDrive APIs and Live SDK for accessing SkyDrive content and services from apps on multiple platforms, the ability to embed tables, ranges or entire spreadsheets in your own web pages as the basis of mashups should see that traffic increase significantly.