It's a measure that makes perfect sense for conventional electricity production: 10,000MW of aggregated coal generation can hypothetically produce 10,000MW more-or-less indefinitely, as long as it keeps being fed things like fuel, water, and maintanence.
But it doesn't make any sense at all, by itself, for energy storage: A net 10,000MW battery might be able to produce 10,000MW, but for how long can that output be sustained? Unlike a group of coal plants, it absolutely cannot do this indefinitely; at some point, that battery will become completely discharged.
It takes at least two figures to describe a working bucket of energy (whether that bucket is Lithium cells or pumped storage or whatever): The capacity (megawatt-hours is a fine figure here, and units like Joules also work), and the maximum input/output (and plain megawatts works fine for this part). Using only one figure doesn't really describe anything at all.
I don't know when or why we stopped doing this, but it's misinformative in a way that leads to a bad generalized understanding of the these concepts with the populace that is actually paying for all of this stuff.