Using "Lest"

 Using "Lest" and not "Otherwise"

The United Kingdom fell silent at 11am on Friday 11th November to remember Armistice Day, the end of the First World War, and to pay respect to those who gave the ultimate sacrifice in that conflict and others.

As noticed in the picture, the sentence has the powerful element of the English language, LEST.

It is used before a consequence clause (phrase) that shouldn't happen, that should be prevented from happening.

So "Lest we forget" means "So that we don't forget". 

We are doing this (celebrating Armistice Day) to avoid forgetting about it. We shouldn't forget.

It's not only used for these very important events... We can use it with our children, for example:

"I understand you are going to sleep at your friend's house. But call me the moment you arrive lest I get worried."

Or:

"Lest she BE mad, you must tell her the truth. These pranks are starting to get out control..."