What is the point of 'Send as SMS'?

Send as SMS, if I understand your question correctly, allows you to save a small amount of data per iMessage sent. iMessages are sent via cellular data, while SMS (Short Message Service) travels along the same route as phone calls. The typical amount of data used per iMessage sent is around 10-20 kB (kilobytes) but this should not be a problem. It can often resolve issues regarding iMessage services being down or attacked at a certain moment (no, Apple can't stop everything but they sure as **** can pay for damages) so in my opinion Send as SMS is a good feature to have.

iMessage only works between other Apple devices that have the feature turned on and are signed in. Sending a message from an iPhone to an Android phone does NOT use iMessage but uses SMS instead. Sending a message from an iPhone to another iPhone that does not have iMessage turned doesn’t work either but uses SMS instead. iMessage is an Apple proprietary system limited to only to other Apple devices.

You can tell whether a message was sent using iMessage or SMS by the color of the text. Blue means iMessage was used. Green means SMS was used.