You do not have to click the install button, however, it feels good for a coder to know that people found his/her hack usefull and people are installing it.
It's a nice way to thank a coder for all there hard work. I click install on every hack I use, because I would like others to do the same with my hacks.
I mainly do this so that I can easily find what hacks I have installed. And it is also nice for the coders to know how many people have installed they hack.