Why would you want to use the Zerg burrow? It doesn't have any combat value for the burrowing aliens. Is it just a defense/run away card until you can set up your own attack?

Yeas, Burrow is good for keeping units alive so they can attack later but it has limited use on the attack.

There are some cases, when Zerg lose lots of his units during rush. Burrow can be life-saving then.

In combo with something like spawn broodling (or Splash Damage) you can totally mess your opponents tactic and save good cards.

Even voluntarly withdrawing could be a tactic to take control of an empty area before your opponent is forced to retreat.

You can force your opponent to play scanner sweep or buy observers , to avoid him playing a better start of battle ability (because you can only play one per battle)

And burrow cards have reasonable defense values : 5 and 6