How many times have we heard this in the BDSM community? The idea is that if a dominant starts out as a submissive, they can better understand what the experience is like for their submissive, which makes them a better dominant. Some dominants even seem to think it's a requirement before you can be a dominant.
I think the whole notion is bullshit. This idea also has some arrogance behind it, because it seems to imply that the person supporting it thinks that everyone experiences things in exactly the same way they do, which of course is false.
Men and women experience things differently. Even if you've had some submissive experiences, doesn't mean your experiences will be the same as his. (Or even hers, if you're a female dominant who likes female submissives).
Not only that, but each individual submissive will experience things differently. You can do the exact same thing to 10 different submissives, and each of them will experience it differently. Having experienced it yourself will tell you absolutely nothing about your submissive's experience of it. Only they can tell you what the experience was like for them.
Unless the dominant has rocks in their head, they'll understand why the submissive is enjoying what they're doing. You don't have to be whipped to understand that a masochist likes pain. You don' t have to be tied up to understand that some submissives like being tied up and feeling helpless. You don' t have to cross-dress to understand that some submissives like to cross-dress. And in fact, if none of these things are your kink, the experience will tell you absolutely nothing as far as what it's like for the submissives who do like it.
If someone doesn't like chocolate, eating chocolate won't tell them why other people like it, or what it's like to like chocolate - because they don't like it! The same applies to starting out as a submissive in order to become a better dominant.