I am having a hard time knowing when to use a reflexive pronoun before a verb. I understand when they are used before reflexive verbs but I often see them in front of non reflexive verbs too.

For example, here is a paragraph from a story I had to read for my Spanish homework.

> 
> La tradición de día de los muertos tiene su origen en una celebración indígena. Se celebra principalmente en Mexico y Estados Unidos 
> 
> 
> 

Questions: 

1. Why do you have to put "Se" in front of celebra- "Se celebra"?
2. How does this change the meaning compared to if you just wrote "celebra"?

Another example,
What is the difference in meaning between: Sí te puedes y sí puedes 

Gracias