if you invest extra money, its more likely that you won't touch it for the near future, lots of people invest money then after a week or a month they withdraw it since they needed money, that is not a good idea, investing money that you will think more often, should not be invested, because, you will lose money that way, but if you use those extra money there is a good chance of a profit, in the future.
Many people 'invest' money with the same knowledge and expectations as they would go to a casino, meaning that they are not really investing, they are gambling. The basic rules for investing are clear: do it with money that you do not need in the short or medium term, have knowledge about what you are investing in, and have an emergency fund before anything else. Following that, we can talk about investing; anything else is gambling.