Flash had been the precedent when it came to advanced animation and interactivity. The use of vector imagery through Flash allows web producers to create some very impressive visuals. The issues with Flash is the reliance upon an application extension. It is something that customers or users do not always have. With the growing population of mobile users this has become more of an issue.
Google has now made it much easier for us to take advantage of these tools with their centralized set of libraries. A developer can use these libraries knowing that users will experience miniscule load times. The benefit is that a user may already have these files cached from another site.
If one decides to use Flash on the web, use SWF Object to implement. It is optimized and more flexible than other Flash embed methods (read more). Also be aware that Microsoft has an alternative Silverlight.
Further reading: SWF Object Transparencies