If the fixed connection fails, for example due to roadworks, a router with a backup connection automatically switches over to the mobile network. This is also called failover.
The advantage: you stay online without anyone having to do anything. The disadvantage: capacity is limited and the speed depends on mobile coverage at that location. For email and the most important systems it is often enough. For a whole office that is on video calls all day, it usually is not.
A stronger solution is a second fixed connection via a different route. See redundancy and dual entry.
What does this mean for the IT-Label?
A backup connection counts towards Connectivity (redundancy and stability) and Infrastructure, where backup connections are explicitly mentioned. A backup via the mobile network is better than no backup. For the highest levels, a second fixed connection carries more weight: IT1+ assumes redundant fibre. See also redundancy and indoor coverage.