Getting your students on your side is important for many reasons. Since you are likely not quite as awesome as a substitute as I am I shall list them for you
1) You do not want a chaos in your classroom. Just think what the other teachers will share about what happened when you took over for the classroom down the hall. Plus, forget about the teacher... what will the principal think? Do you want a job? Then for the love of everything impress the principal!
Sidenote: also impress the secretary they know everything!
2) Do you want to be invited back to that class? I thought so... get the kids on your side so that when the teacher tells them she needs to get a sub to cover for a PD day they will all chant your name and not that other substitute who thinks they are all that and a bag of potato chips (because you are NOT them).
3) When the kids are on your side they will do things for you! Want a foot massage? Want them to clean up the room at the end of the day? Want them to get that math worksheet done so that you don't have to fill in all the answers in kids writing to make it seem like the students actually did it? I thought so... get them on your side.
As you can plainly see you want the kids on your side and as far away from the side of that other substitute. Luckily for you that substitute is not stylish at all. If it is a guy he wears plain clothes and not a cool watch. If it's a girl she has brown or black shoes every time and no jewellery.
Now, I'm not saying spend all your money on some new clothes and accessories... that's crazy talk you are only a substitute teacher and you do need to eat. But trust me, some red flats, bracelets or a ring, and a cool watch go a long way. I honestly can't explain it but a kid will comment on my bracelet and the next thing I know they are giving me a hug and asking if they can fan me... ok I've never actually been fanned.