The short answer is because you might end up recording over something you want or need.


The long is answer VERY long and complex.


Every person has different needs and different circumstances so I suggest researching the issue and basing your decision on your own needs.


With modern large capacity memory cards and the ability to record in reduced resolution, I see no reason why you should use loop mode and risk recording over something.


Remember that loop recording was the mainstay when VHS and similar tapes were used and they had like 35 minutes to 1 hour of recording time.


Today you can easily get 14 - 20 hours out of a single memory card.