Back when the internet was a commodity that only very few people had, and buying games online was something that nobody had yet even dreamed of yet, and thus all video games were sold as physical discs in stores, it sometimes might have made sense to pre-order certain games.
After all, there were only so many physical discs that a video game store could have in stock at one time. If a game was expected to be immensely popular and you really, really wanted to get it on day one, pre-ordering the game made sure that you would indeed get it. While most other people were walking out of the store empty-handed because the game was sold-out, you would simply walk into the store and ask for your copy of it, which you had reserved. Or even have one sent to you by mail. People who didn't pre-order the game would need to wait for days or even weeks before getting their copy.
Not that this scenario was very common, but I suppose that in some cases it could have been.
Since every single game nowadays is sold online (some of them even exclusively online), pre-ordering doesn't make any sense anymore. If you want the game on day one, you'll get it. Just go to Steam, or whatever platform you are using, and buy it. It's that simple. It's not like they will run out of copies to sell.
It would make more sense if there were some kind of good incentive to pre-order. A great incentive would be to get it cheaper. For example, normal price for the game is 59.90€, but if you pre-order, you get it for 49.90€. A whole 10€ saving is a good deal for a brand new game.
But no. That is almost never the case. I don't remember seeing pre-orders being cheaper ever (there might have been some cases, but they are really, really rare.) You might get some completely inconsequential in-game stuff with a pre-order, but that's it. Most often what you get is completely useless and not really worth it. (In a few cases it might, quite ironically, even be detrimental for your playing experience. Such as getting a load of in-game currency, or some high-powered weapons from the start, which in some games would lower the challenge of the game.)
There are many reasons not to pre-order.
One is, of course, that you are buying blindly a product without knowing if it's good or not. Too many times have games been hyped to the heavens, and tons of people have pre-ordered them, and then it turned out that the game spectacularly fails to meet those expectations, and is considered if not an outright commercial disaster, at least bland and average, and in no way worthy of the hype. In other words, something that most people might be willing to buy if heavily discounted, but not at full price.
The other reasons are subtler.
Pre-orders entice game developers to rush the publication of their games. (Or, more precisely, they don't give them much incentive to polish their games before publication.) More and more games are published unpolished, full of defects and bugs.
In most cases these defects and bugs are fixed with subsequent patches, which is great and all, but the problem is that you, as a first-day purchaser, are experiencing the first buggy version of the game. Things might not look like they should, the game might not function properly, and there may even be crashes and other problems. You are experienced a flawed version of the game. People who buy the game weeks or months later will be experiencing a better version of it.
Do you really want to experience a buggy version of the game, while others are experiencing a much better version of it?
If people stopped pre-ordering games, and rushing to buy them as soon as possible, then perhaps game developers would make more sure that their product is as flawless as possible, so as to not get crushing reviews from critics, which would discourage people from buying.