What kind of use cases require a 2GHz, 4GHz, 6GHz, 30GHz or 100GHz oscilloscope?
Systems where you needed to debug, test and validate things that operate at high frequencies would require scopes with those higher bandwidths such as:


USB2.0 
USB3.x
6G SATA
PCIe Gen 3
PCIe Gen 4
1G Ethernet
10G Ethernet
40G Ethernet