Or for third parties to bundle "for free."

For example Walmart+ bundles Paramount+'s ad tier. T-Mobile currently bundles the Ad-Free Basic/Standard depending on wireless plan, maybe they'll move to the Ad tier.

My guess is it's for packages where you get Netflix included with something else. For example, you sign up for Verizon and get Netflix for 6 months. Now they have a cheap version of Netflix to give you.

The old mackbook strategy

People who got six months of a Netflix subscription as a Christmas gift and got partway through a series,

Correct. This is the start of increasing prices and testing ads on other tiers.