Why is computing of Quadratic variation or knowing about it is important in Stochastic Calculus (for Financial modelling)?

I am reading Steven Shreve's Book 2 along with the help of a Phd Mathematics student.

I have watched several videos on the above topic, where I am not clear/or struggling to register in my mind is 'what is that I am missing if the above is not computed'

Be it in Itos integral, jump process or the entire book of Steven Shreve's Stochastic calculus.

Thank you very much in advance for all the time and knowledge.