The difference between 'at once' and 'all at once'

English is an interesting language. For one thing, the word need not be pronounced as it is spelt. For another, the same word can be used as different parts of speech. Sometimes the addition of one word in front of two existing words can mean different thing.

For example, ‘at once’ means ‘immediately’ as used in the following sentence:

The waiter came at once when I beckoned to him.

If we add ‘all’ in front of ‘at once’ to form ‘all at once’ the meaning is totally different. ‘All at once’ means ‘Suddenly’. The following sentence carries this meaning:

At at once, it rained and I was not able to find shelter in time and was drenched through.