I understand the confusion.. let me try and explain..
But.. Why disable it..?
The reason behind disabling it.. Well.. Let me first kind of define an administrator account in a very generic way:
It is a user profile that is had administrator level privileges enabled which essentially gives that particular user complete access to the system.
So.. On windows it is named as 'Administrator'. It always exists on the system, whether enabled/disabled/in-use or dormant. So.. when malwares are written they are programmed to check for the existence of such profiles. Someone could also rename it to admin or root or something like that are generally privileged profiles.
If they do exist.. Voila... It saves me the work of "executing additional code" to gain privileges in the system. People make mistakes.. If they are enabled, let's take control and play havoc.
To make it more clear, let us assume that you are a skilled burglar and want to break into a house. Assume that the "house key" is the admin account. If I check for the house key in generic places like 'under the pot, or under the welcome cat, or on the ledge".. If I get them, I'll break in easily. If you don't keep the set of house keys outside, then I don't find it. The burglar will break it using additional pick-locks and levers (Privilege escalation methods). If your lock is unpickable then you are safe. (Relate it to something like hardening your security).
So you see, disabling it will ensure that the malware atleast has to "put in work effort" in order to gain access to the system. A highly motivated and skilled hacker might be able to get through but can be stopped by implementing tight security controls on the system. If your system is secure, then the malware is useless.
Theoretically speaking.. I hope its helpful.. :)