Integrated free into the base game, Together for Victory gave the British Empire's dominions an identity of their own, when until then they were little more than a voiceless extension of the UK.
Nations and Content
Canada, Australia, New Zealand, British Raj India and South Africa got their own focus trees, each with its own — more or less tense — relationship with the metropole.
What Changes
It introduces the autonomy system: dominions start with different levels of independence from the UK, and can gain more over time, up to full independence in some cases. It also adds technology sharing between countries in the same faction, something that didn't exist in a structured form before.
Is It Still Worth It?
Free of charge, it's essential to properly understand any UK or dominion game — the autonomy system is still the foundation of how relationships within a colonial empire work in the current game.