Okay, I think I better understand what you're confused on. The "asset" you are describing with the mailbox icon actually isn't an asset at all, it's a program. There are four kinds of programs: Engagement, Event, Email, and Default, which all serve their own purposes. I'll use an analogy to try to clear things up.
Think of programs as large file cabinets. You can have the four different kinds (listed above), and they each represent a completely different part of your "business" ( aka your marketing strategy). They encompass everything that has to do with a specific marketing effort. The drawers are one way to use folders, i.e solely for organization purposes. Inside the drawers (folders), there are the hanging folders (a sub-set of Marketo's folders), also solely for organization purposes. Inside of those are your local assets (emails, LPs, forms, etc.).
Overall, you want to use folders to the extent where everything is organized. Assets are completely unrelated to folders, other than the fact that you can store assets inside of them. Programs should contain all of the above, as they need it all to operate correctly.
Yay or nay?