It is a small act of contrition and penence in addition to being a reminder of Jesus’ sacrifice for us. The US Latin Church no longer requires this, however one may still do it if one chooses. What is still requred is some small act of penence still be performed on Friday, wether that is prayer, Mass or something else. That is my understanding at least.
Its a fast to help us appreciate and partake in the sufferings of Jesus and remember every Friday that He did die for us. As for weather or not you have to do it i think Conferences of Bishops can receive permission to allow people in their nations to substitute another thing or add more prayer or something but that would depend on the Conference and dioceses. But there are Days that are Mandatory (Ash Wednesday, Fridays of Lent and Good Friday).
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