For a desktop, this can be transparent for the user, because it can be booted via Ethernet or from an USB drive that is attached all the time to the computer, possibly on one of the internal USB type A connectors that exist on many motherboards, precisely for this purpose.
Using a single disk with multiple partitions is less convenient than using a separate boot drive, because the separation makes easier the reuse of both the boot drives and of the root drives in other computers, or their copying onto drives of different sizes, when migrating or cloning operating systems.
For a laptop that contains encrypted SSDs/HDDs, using an USB drive to boot it, which is not normally kept with the laptop, can be used for improved security, because only in this case no secret keys need to be stored on the encrypted drive. Even if the secret keys are also encrypted, they must be encrypted with a key derived from a password, which can make their decryption much easier than the decryption of a drive encrypted with a random key.
Using a single disk with multiple partitions is less convenient than using a separate boot drive, because the separation makes easier the reuse of both the boot drives and of the root drives in other computers, or their copying onto drives of different sizes, when migrating or cloning operating systems.
For a laptop that contains encrypted SSDs/HDDs, using an USB drive to boot it, which is not normally kept with the laptop, can be used for improved security, because only in this case no secret keys need to be stored on the encrypted drive. Even if the secret keys are also encrypted, they must be encrypted with a key derived from a password, which can make their decryption much easier than the decryption of a drive encrypted with a random key.