So, Pets have Energy just like the player does.
And to cast Pet Skills like Briarthorn’s Ground Slam or Familiar’s Lightning strike for example, they need to have enough Energy to do so.
How much Energy Pets have scales with Player level.
Pet Energy becomes pretty much irrelevant at level 100 since they are never going to run out of it.
But, during leveling, if you were to say go for a 12/12 Lightning strike on Familiar as soon as you have the available points (which should be in the player level 20-30 range), you will find it constantly running out of Energy to cast the skill.
Perhaps Pet Energy regen can be increased to make it easier or give them Energy scaling with Skill points invested into the Pet itself.
For example, having a 16/16 familiar running out of energy after 4-5 casts of lightning strike atm, only serves to make the player resummon the pet to restore its energy rather than have any meaningful impact either way.