Setup may refer to any of the following:
1. Setup is the overall process of connecting and preparing a software program, hardware device or computer to function properly.
2. Setup is a file commonly called "setup.exe" that is executed to begin the process of installing a software program onto a computer.
Why do programs need a setup?
A setup program makes it easier to install a program on your computer by creating the directory that stores the program, extract all of the needed files, create shortcuts, and change system settings and permissions needed for the program.
Without a setup program, it would be up to the user to perform all of the steps that the program needs to have done before it could run.
Should I use "setup" or "set up" in my writing?
When being used as a noun (e.g. see the setup instructions) it should be one word. When being used as a verb (e.g. I just set up my computer) it should be two words.