1] why is it important to apply ‘PACC’ when communicating in different social situations ?
You would know what tone to use, depending on whether you are angry or happy [etc]. You would also know how to phrase your sentences depending on whether the conversation is formal or informal, and the background of the situation and thus knowing what kind of tone to use.
2] If a majority of your classmates hold a certain view about a controversial topic, while you hold an opposing view, how would you convey your thought politely ?
I would calmly explain to them why i have such an opinion and hope to have their understanding, not forgetting to use polite language and a nice tone. If my opinion really is incorrect, i would not oppose further and would gladly agree with my friends.
3] What are the possible repercussions if the tone and register were used wrongly ?
The other party or the person you are conversing with might have the wrong idea that you are angry with him/her / [if conversing with a boss] not professional enough and not serious about the situation [ supposed to be formal but instead used informal language ] / [guest] not welcoming him/her [etc]