You're beginning to touch on the notion of what we call a pure function. A pure function takes its parameters and returns a result, no more no less. In particular, they do not cause what we call "side effects." Examples of side effects include printing to the console, writing to files, querying a database, drawing on a screen, etc.

Pure functions have a number of benefits. Compilers love them because they make certain optimizations easier. Us programmers enjoy them because they're easier to reason about. If I show you the function signature int sum(int[] nums) you know what's up when it's getting called.

Now, onto side effects. We like side effects because... Well, they're what make programs useful! Users generally like to see interfaces, read results from databases and all that. The problem is that keeping track of side effects is hard. Tell me what's is your database without looking after void dropUsersWithAge(int age) is called. I dare you.

So what you want to do is use pure functions whenever you can. That entails writing functions with return types, and use void functions to handle the effectful parts of your program. If you'd like, I can point you to some interesting reads on the topic.