Actually I don't even think connecting AWS services to each other is a good reason in most cases. I've seen too many cases where things like this start off as a simple solution, but eventually you get a use case where some s3 updates should not sync to dynamo. And so then you've got to figure out a way to thread some "hints" through to the lambda, either metadata on the s3 blob, or put it in a redis instance that the lambda can query, etc., and it gets all convoluted. In those kinds of scenarios, it's almost always better just to have the logic that writes to s3 also update dynamo. That way it's all in one place, can be stepped through in a debugger, gets deployed together, etc.

There are probably exceptions, but I can't think of a single case where doing this kind of thing in a lambda didn't cause problems at some point, whereas I can't really think of an instance where putting this kind of logic directly into my main app has caused any regrets.