So, Tasha's Cauldron of Everything introduces a lot of new "pet" mechanics in DnD 5e. An emerging trend for some of them is that the master can take a bonus action to command the pet to take a specific action. For example:
Circle of Wildfire Druid's Wildfire Spirit
In combat, the spirit shares your initiative count, but it takes its turn immediately after yours. The only action it takes on its turn is the Dodge action, unless you take a bonus action on your turn to command it to take another action.
Beast Master Ranger's Primal Companion
In combat, the beast acts during your turn. It can move and use its reaction on its own, but the only action it takes is the Dodge action, unless you take a bonus action on your turn to command it to take another action.
A Pact of the Chain Warlock with the Investment of the Chain Master eldritch invocation
When you cast Find Familiar, you infuse the summoned familiar with a measure of your eldritch power, granting the creature the following benefits:
- As a bonus action, you can command the familiar to take the Attack action.
While it probably won't matter much for the Wildfire Spirit or the Primal Companion since they both take their turns on or right after the master, a familiar has its own initiative and acts on it own turn. So, if a Pact of the Chain warlock with the Investment of the Chain Master eldritch invocation uses its bonus action to command their familiar to take the Attack action, when does said familiar actually take the Attack action?
Contrast this with the base Pact of the Chain feature, where a familiar can use its reaction to take the Attack action on the master's turn, should the master forgoes one of their attacks:
Pact of the Chain
Additionally, when you take the Attack action, you can forgo one of your own attacks to allow your familiar to use its reaction to make one attack of its own.