Macros have signatures for security reasons. Creating macros is great and a powerful tool in Kentico. Problem is it opens a lot of holes for security issues which is why they have signatures. When each macro is they use a hash key which in v8 is now a web.config key. In v7 and earlier they used the connection string which changed from environment to environment (dev, staging, prod). When you change the hash key, the signatures become invalid and need to be resigned, which is what you did.
You may not have even used a macro but there are macros used all throughout the site that you may not even know about, especially while working in the UI. If you haven't read up on macros, I'd suggest using the page Kentico provided and check them out.