Carrier Aggregation: The more bands the repeater has, the more carrier aggregation (CA) can take place. CA is when the operator combines 2 or more frequency bands to provide the user with much higher bandwidth and data speeds. One of the core technologies of 5G, which enables it to perform at these super high speeds, is CA.
Redundancy: The more bands you have, the more robust your system will be. Even if one band is compromised, which can happen a lot, the phone can still use the other bands.
Future Proofed: The more bands the repeater has, the more likely the system will function well in the future. The radio frequency landscape changes all the time. Think of the transitions from 2G to 5G. Limiting yourself to just one frequency means you are less likely to benefit from new improvements that the operators make to their networks as time goes by.
Covering Natural Signal: It is very important that the “natural signal” in your building is “covered” by the repeater. Mobile phones always try to connect to the highest mobile signal that is available. Lets take an example of a house that has installed a single band 900MHz repeater. If 1800MHz is available in this house, even it it is very weak, the phone will bypass the repeater completely, and connect over the weak 1800Mhz signal. By installing a multi-band repeater you can reduce the chance of this happening. Ideally we would just install a 6 band repeater and not have to think about this. However, the cost is prohibitive for domestic installation, so a dual or tri-band is a good compromise.
