I can explain more.
For example lets say i want to serve on my desktop 15 of my latest articles.
First scenario: If i want my index to have a top slider where it will show 3 latest articles and for the rest 12 articles i will use content listing module to show them it will show again those 3 first articles. If there is offset option on the module i can set it to 3 so the module will skip the first 3 articles.
Second scenario: If again i want to serve 15 latest articles and use three times the content listing module and each one will show 5 articles with a different module layout. Also i want between those modules to add advertising block. Also this case is only possible with the offset option.
If you didn't get my point please let me know to try to explain it with images.
Kind regards