Normally, you can only play units during your deployment phase. The Dragonsnake can be played any time there is an action window, including on your opponent's turn. Remember, there are action windows after each player refreshes, before each player draws to their reserve value, before engagements are declared, after attackers are declared, after defenders are declared, after each strike, after each engagement, and after the force struggle.

So why would playing the snake outside the deployment phase be useful? You could do it after an opponent declares attackers to gain a surprise defender. Alternately, you might keep the snake in hand to protect it from tactics or events and reactions that place focus tokens, such as Cloud City Operative.

In a few cases, you might also wait to play the snake to manage your unit count. This could keep your opponent from playing units if he has an undamaged Hidden from the Empire. It could also keep your opponent from triggering the Kashyyyk Resistance Hideout's shielding effect or gaining extra combat icons from Hold-out Blaster.

Finally, you might wait to play the snake to protect it from board wipes such as My Ally is the Force or Force Storm.

Exactly. Also, I have seen many players use similar "type" effects to protect units or use resources. You can hold back those 2 resources for a potential (or bluff) Rage, and then play it in their Force Phase.

For those of us who played a lot of the 3v1 decks, this effect is pretty useful.