I’m going to mostly agree with ab and peegee, but with a little diff spin.
First - as pointed out previously, there is a difference between Linux and Windows (idk Mac). Shutdown or restart can be used to refer to a program OR the computer & OS. Imo, your young friend was almost certainly referring to the Windows world, but it’s not as different in the Linux world as many would like to believe.
When shutting down or restarting the box, meaning the complete OS, there are several reasons to wait before restarting. They have changed somewhat over the years, but there are still valid reasons to do this.
First, you have hardware wear-and-tear. Shutting down and rebooting mean a change in the state of moving parts. Even at the micro stage, this creates a wearing effect. This is probably the least important reason to wait, but there it is, anyway.
Moving right up the scale, you have a latent affect in memory and other electronic parts. When I got my start with PC’s, I knew a lot of computer people who denied this that there could be a “RAM carryover”, but I have seen it operate way too many times. It is real. Hardware and the OSes have greatly improved, but this has only shifted the issue. I almost never see RAM carryover anymore, and I think it is because of hardware improvements, but it could also be because I almost never immediately restart. When I started, the possibility of RAM carryover was very real, especially when there was an issue.
Today we have the soft “off” referred to, in addition to reset switches. And, the power switch on the front is no longer a real “on/off” switch for the power. The motherboard will have power so long as the box is plugged in, and it has a battery backup, jic. A variation of the memory carryover case that we have today is that, so long as there is power to the box, there are settings retained on the motherboard. I’m writing this on a Ubuntu machine that will not reboot if I shutdown, until I completely remove all power from the motherboard (power cord and battery) for something like 15-30 seconds. Long enough for the bios to lose its startup settings. Since this machine is also a home server, it almost never shuts down. This means this is not a problem, so I’ve never fixed it. I’m sure it could be fixed, but the point is that SOMETHING is still being retained, from the OS, by the mb.
In the Windows world the possible memory issue alone would be enough for me to still advise people to give it 10 seconds or so after shutdown. Windows is designed so that it relies on a full shut down to reload the registry, drivers, and whatever. If a Linux OS were badly shutdown, I think you might want to keep the same response. These previous posters are going to know more than I about the full hardware implications of a Linux OS shut down, whether shut down cleanly or not.
So, your young acquaintance was quite likely repeating good advice, at least for installing software in the Windows world: “You’ll find you’ll have a lot fewer problems.”
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