How are SQL backups different from IT regular nightly server backups?
Database backups give you point-in-time restore capability (provided you have FULL recovery model). Even if your IT people take backups every few minutes, which is extremely unlikely, you'll still have a gap.

Server backups do not replace database backups, they complement them by "archiving" the database backup files long-term (i.e. more than just today).

In the end you and your management have to decide your RPO (the recovery point objective - how much you need to be able to recover in a crash). With only daily server backups and no database backups, you stand to lose a full day's work in the worst case.

Edit: @Sting has a valid point in that shadow copies (the mechanism most likely used to make server backups) are not very likely to take an exactly simultaneous copy of all your database files (including log files), which may lead to inconsistencies when you restore the backup. For instance, if the shadow copy reads the transaction log a few milliseconds before it reads the database file, the database file could contain an uncommitted transaction, but as the transaction was committed a millisecond later, the log will not have any record of it.