The fact that the first person you mention is "your best friend" is an indication of the importance you're placing on it. Nobody is using a nom de plume for their electric bill, you're being deliberately obtuse.
A best friend, by definition, is one of the people you choose to trust the most. If you want to replace it with someone else significant in your life, by all means, it doesn’t change the point in any way shape or form.
A random company is the other end of the extreme, it’s an example of a very limited relationship.
The point is that the bigger your relationship, the more you’d entrust them with secrets. Secrets like your real identity. It’s a pretty basic and obvious concept.
Your best friend? I would hope you feel comfortable enough sharing your RL identity or deeper secrets.
A random company you interact with? Why would it matter, let alone be one of the most important things?