Causality ensures that physical processes have causes
Causality in physics is actually quite simple: a cause precedes its effect, and an effect cannot occur before its cause.
This means that events are ordered in time, and the influence of one event on another propagates through space over time in a consistent manner.
Cause and Effect
Essentially, there needs to be a limited speed of propagation in order to preserve cause and effect relationships. Having no limit of propagation speed would lead to paradoxes where effects could precede their own causes, or happen simultaneously.
Without cause and effect relationships, the universe would break down as no processes could predictably happen.
Principle of Locality
An object is directly influenced only by its immediate surroundings. Physical interactions cannot happen instantly, but need to propagate through space.
This ensures that an event cannot happen without a cause.
Speed Limit
We know that a speed limit must exist, but what sets the actual number of that speed.
Why is the speed of light what it is?
It is a fundamental part of how spacetime is structured.
When you tip over a line of dominos, the falling dominos travel at a certain speed.
Free space, a complete vacuum, has a similar property with how it responds to electric and magnetic fields.
Due to Locality, these fields can only interact with what is immediately surrounding it. This is similar to how a domino can only cause the next one to fall, not one randomly on the other side of the room.
The speed at which an electromagnetic wave can move through the ‘dominos’ of spacetime is c, the speed of causality. aka: the speed of light.
ps If you don’t know how a field can exist in a complete vacuum, consider reading Quantum Field Theory.