When periodic backup misses its last scheduled run, it's now possible to tell the app how to handle it.
There are two common cases - (a) the app itself wasn't running and (b) the backup drive is a removable and it wasn't plugged in.
With certain setups it makes sense to run the backup as soon as the app can, to "catch up" with any changes and get them backed up quickly.
In other cases, a backup may be large and heavy and set up to run every midnight so not to disturb anyone. Running it ASAP will generally be not a very good idea.
Hence this option.