Another dimension to this is that intelligence agencies, by their very nature, are very difficult to have proper oversight and regulation of. So it may be in a nation’s best interest to have multiple separate agencies that don’t really compete with each other and have independent chains of command so that one agency doesn’t unilaterally control everything and it’s more difficult to compromise your intelligence network. See how in the US the NSA handles mostly electronic and signals intelligence while the CIA deals more with human agents and spies, and the NRO handles space assets and satellite based intelligence that it provides to those agencies and the many others in the US intel network.