Is it necessary to temper chocolate when making candies that will be eaten that night? What precisely is the reason for it?
It makes the chocolate nice and glossy and gives it that snap when you bite into it. I would do it, regardless of when it's going to be eaten, as Zeldog said. It's much easier to do than you'd think.
Chocolate is an unusual product which can solidify into different crystalline states. When you "temper" chocolate you are coaxing it to solidify into one of it's more stable ones. This gives as chowser mentions a nice shine, snap and a smooth mouthfeel. Otherwise, it can be soft, grainy and have blotches on it.
If you make filled chocolate, (truffles or bonbons) they will not come out correctly if they're not tempered (ooze or collapse, or not come out of the mold) while chocolate bars will melt quickly in the hand or be very soft.
Overall it will be a much nicer experience if you temper the chocolate.