Scripts are designed to automate tasks on IRC as quickly as possible to overcome human limitations. Anything that you can do with a script on IRC can be done by hand.
For example, using a script, you can automatically kick users from your channel who swear. You can also use another script to tell you when a user was last online. Both of these are tasks that a human can do, but it would take much longer to do.
Some scripts break that “mantra” by adding functionality that does not automate a task, but rather improves your experience. Such scripts are usually theme scripts that changes the way your client behaves or looks.
Beware: Some scripts can be dangerous. Never load scripts you aren’t familiar with.
It goes with out saying, scripts are executable pieces of code that run on your computer. Even though they’re just mIRC scripts, they can still have harmful side effects if they’re used improperly. You wouldn’t (hopefully) run a random .exe file, .dmg file, or .deb file on your PC, and you probably wouldn’t install a random .apk on your phone. So why would you load a random .mrc file into your IRC client?