Every Marketo program must have a channel defined so that you can properly track the progression status/steps (and measure success) as leads engage with your campaigns (e.g., "Email w/Landing Page" channel could include the following progression steps: opened, clicked, bounced, unsubscribed, filled out form, engaged/downloaded [success]).
Tags are useful for reporting (primarily within Revenue Cycle Explorer). We use tags extensively to define the program, campaign, area (NA, Europe, APAC), region (country), content type (whitepaper, ebook, research, etc.), Major Event (Sapphire, Oracle OpenWorld, Convergence, etc.), industry, service line, etc. Each time we setup a new program in Marketo, we need to specifiy the values for each of these tags. A bit tedious, but well worth it when it comes time to slice and dice our program effectiveness/attribution.