Why do I need restart computer after installing new software?

Why is it sometimes that I need to restart my computer after installing new software and at other times I don't?

Is there any reason why it needs this reboot or why it's not always one way or the other?

It depends.

If the software being installed affects an integral part of the operating system then a restart is required. For example a new kernel for the operating system.

On Windows systems, it is often used used because users are considered to be too stupid to use their computers properly. As an example, Microsoft publishes details of how to change the "Node Type" used for networking on its website, including the instruction to "restart the computer" when all that is required is a restart of a network service as detailed here. Because we, as users are too stupid to restart a service, we are told to restart everything.

For some pieces of software, I have come to the conclusion that it is a habit and is often not necessary even when told to do so. If I don't think that a piece of software should have done anything major to the operating system, I tend not to bother, and haven't experienced any problems (and if there were problems they would be easy to solve).

Sometimes a piece of software will make a change that cannot come into effect while the computer is use. Some reasons might be - a file is in use, the change can only occur during boot up of the computer, there might be a security issue which can only be done before the computer has its networking active, maybe the virus scanner would interfere with the install.

Sometimes, it's just sloppy programming by the developers.

I'm sure there are many more.