Command grouping in sed

Question

I do not understand the command grouping in sed scripts. We use curly braces to group commands. I found some information in the first answer to the following question:
Using multiple sed commands .
But I still do not understand this properly. Could someone please explain this to me?

Answer 1

If you use

/Number/ s/N/n/;s/r//

Then r s will be removed on all lines, not only those containing Number . But, if you use

/Number/{s/N/n/;s/r//}

then r s will be removed only from lines containing Number .