Whether or not there's any security benefit, I've often benefited from pre-reading install scripts by deciding to install the thing somewhere else, or with a different setting, or noticing that the entire install script is basically just a lot of paranoid noise around simply downloading the binary and putting it in a binary directory, etc.

Some are so elaborate it's hard to poke through what they're actually doing, but a lot of them aren't.

Making an all-in-one install script that doesn't require any arguments involves making a lot of decisions for the user that you may not particularly like. Pre-reading gives a chance to decide whether or not you like those decisions.