Healing Drone is a "Support" unit, his main role is to help sustain his alies.
Passive: Got Your Back
Heal units around him
Active: Just Keep Breathing
Restoring health to all attack units on the field
Tactics:
passive - healing units around him making them last longer, this effect goes really well with "Tank" units making them last longer + to frigile units healing the from AOE damages.
healing drone are trying to scatter in the wave, meaning more healers better coverage.
active - use of "Just Keep Breathing" on the right time can make the difference between successfull attack and failing one.
using the skill to soon means that most of the units already have "high health" but using it too late might resolve in the units dying before the use