[General] As vs. when

[constantinusphilo]

This is a quote from one of Winston Churchill's speeches:
It is like setting up an armoured umbrella, under which other countries can be invited to take shelter as and when they seek to do so.
I am not sure I understand the difference between 'as' and 'when' here. One of them looks redundant to me.

 

[tedmc]

It is a commonly used phrase. I think as refers to the reason(s) to do something; when refers to the time to do it.