Power
The "power" of a substitute teacher presents a certain conundrum. A substitute teacher is technically given classroom authority by the school district, but that doesn't mean students are willing to recognize that power.
When a regular teacher says, "Calm down and sit in your seats," students will do it because they have to deal with that teacher every day. When a sub says it, expect a good laugh and a rolled eye or two.
Make no mistake—you'll still have the same disciplinary responsibilities as a regular teacher. You're still expected to maintain a safe, orderly classroom where students are learning. But as anyone who's ever had a substitute teacher knows, it's a lot harder in practice to get students to listen to you.
In reality, subs don't have a lot of power. But that's not the reason anyone gets into education in the first place, is it? Most people don't become teachers so they can sit back, kick their feet up on the desk, and order people around. You're not some classroom Ozymandias; you're more like a Wizard of Oz—a figurehead with a bag full of dreams.
Now, teachers do have a sort of intangible power that can be far-reaching and profound. They're helping shape the minds of the next generation. They're imbuing young people with the necessary skills to better themselves. As the substitute, you're...not really doing that. You're kind of just there for the day to make sure nothing gets too wacky. Enjoy the paycheck.