why is "Live and Let Live" a necessary philosophy to live on Earth?

Because otherwise you will kill and/or be killed by any other more violent philosophy you live by.

Kindness breeds kindness.

Unkindness breeds only death and unhappiness in people's lives.

Another truism would be: "He who lives by the sword dies by the sword".

However, this doesn't mean you don't practice Self Defense either.

It just means you aren't the first to draw a sword and use it. You only draw yours when absolutely necessary when you know you are in danger.