Playtesting Your Own Games
If you are going to publish an adventure (or whole game system), playtesting is critical. Working out kinks or conceptual flaws during playtesting means that allllll those gaming groups that run it later will have more fun at the table.
It takes effort, but it's a huge multiplier of work vs fun: a single hour of playtesting means a better hour of play for every group that runs the final game. If 100 groups play your adventure, your one hour of playtesting impacts 100 hours (or more) of actual play time. You're making a big investment in all those gaming groups having a good time.
If you do the opposite and not playtest properly, you are basically wasting all those gamers' time. Game nights are rare and precious: as a game designer, take an oath to do your best to protect them!
So you know you should playtest. No problem, you say. You ran the game yourself and it went great. Time to release!
Running your own game is not playtesting. You already know what you are trying to say in the text, so you are not really running the game from the document you wrote, you are running the game from the ideal document in your head. You will never notice if you left something out, or explained something badly.
An ideal playtest GM has no idea what you have in mind, has no past exposure to your concept. The playtest GM just receives the document cold, reads it and eventually tries to run it, because that's how every GM who gets the adventure later is going to have to do with it.
Yes, running your own game is a helpful part of the process, but it's really pre-playtest testing. I run a game before even deciding whether to write the full draft that will go to playtesters. Running your own game to work out kinks is certainly better than having no one run the game at all, but it is far less effective then seeing what happens when someone else tries to interpret your words and turn them into a game.
And if you write an adventure but never run it, never playtest it, and then just publish and release it upon the world… well you really should put some kind of disclaimer on it. It should be an apology, because you need to admit that you didn't hold up your end of the bargain.