This was a nice catch by user AD on the Community and something I somehow missed before! It has significant effects on tracking and on branding in general.
When you set up Domain Aliases in Marketo (and corresponding CNAMEs in DNS) you can access your Marketo-hosted LPs via a choice of different URLs. The path/page name stays the same, but you can use a different domain.
For example, you can use go.example.com for the bulk of your pages, but publish some pages under get.mymicrositeexample.com for special products. The pages will actually be accessible under any of your Aliases as well as your Primary (i.e. if somebody manually changed the URL in their browser) but by taking care to link to the preferred domain from ads, social, and email, you can keep content/layout/logos/etc. totally different.
If you're an agency, you might even use Domain Aliases to service multiple clients at the same time, the only restriction being you can't use the exact same /pagename but would have to mix it up like /jiffylube_coupons_2017 and /spiffylube_coupons_2017.
Problem is, “siloing” Domain Aliases from each other falls apart if you select a Thank You page from the dropdown of Marketo LPs.