Why is it important that we do not plagiarize someone else's work?

Because that you be cheating and dishonest.

It is quite OK to use someone else's work as part of your work but you must ALWAYS mark the bits of work you are using and give credit for these bits to the original author. Indeed you will get credit for doing this as it shows you have researched the subject.

However if you pretend that someone else's work is YOUR work, this is dishonest and if/when you are fund out you will be servery marked down or even thrown out of a school/job.

There is software that can tell if you plagiarize work.