Even if you are investing small amount, if you keep investing every month, it will accumulate to a big sum in future. As you are investing every month, the prices will also average out.
This is a very general and non-specific statement, since the future has certain boundaries. It is desirable that they be visible. Moreover, small amount and big sum are very different for different people. Of course, you can set aside "small amounts" every month in the hope that in 50-60 years you will be able to use them, but basically people are motivated by what they can get in the relatively short periods, usually measured up to ten years. After all, you are not only investing your money, but also your time.