Pre-allocating.
You may have already heard about it. It happens on Steam when the game developers push out an update and in order for you to play the game again you have to download and install an update.
Steam is doing it in a way that it will first create a temporary backup copy of each file that is being patched before updating.
When the game is compiled wrongly, for example like Path of Exile is, it can really take hours for the game to update even if you would have an SSD. It's because Steam has to backup this huge single file with pretty much the whole game inside it. It doesn't matter if the update is small or big - the download speed is never a problem.
It was really pissing me off sometimes when I wanted to play for just a little while, but I couldn't because of an update. I really hated that. I actually still don't know if this is Steam's own problem or it's because PoE developers compiled the game wrongly. I don't think this problem happens with other games, but I am not sure about that.
The standalone client of Path of Exile doesn't have this problem. If you want to play and not wait, I suggest you to not play through Steam.
That's all what I wanted to say.
Thank you.