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When I was programming in Pascal a long time ago, we had Functions and Procedures. Functions returned a value, Procedures did not. But there was more to it than that. Procedures had side-effects, but functions generally did not. This idea is called “Command-Query Separation” and it is useful today when writing object methods. Quoting from the Wikipedia:

Command-query separation (CQS) is a principle of object-oriented computer programming. It was devised by Bertrand Meyer a part of his pioneering work on the Eiffel programming language.

It states that every method should either be a command that performs an action, or a query that returns data to the caller, but not both. More formally, methods should return a value only if they are referentially transparent and hence possess no side effects.