Not all companies are like this, and I find it common that new employees are not sufficiently taught what their company is like :
A) you have been performing well as N,promotion means "we feel / hope you're ready to work as N+1 in the future "
B) you've done great as N and have repeatedly performed N+1 tasks successfully. Promotion means "we recognize what you've already been doing "
First one is "proactive" and represents faith you'll do well at next band. In principle all you have to do is do your job well. There is risk though that you man not succeed after promotion if next band has radically different role or skillset.
Second one ensures you're prepared for your new band, but you cannot there just by doing your job well, and many people are unaware or don't have opportunity to do tasks of next band in their current role