You top a plant when you do not have enough height in your grow space or you want more than one main (crown) bud.
(Take note: topping is best done only in the growth phase, although some do it a week or even two into bloom, so if the plant is allowed into bloom and then turns out that space is limited, it can still be topped)
The advantage of topping is that your plant grows wider and not higher, but topping is mostly done because the underlying buds then get more sunlight (better light penetration to the bud), so there’s more weight since as we know, light = weight.
Topping involves nipping out the top-most grow shoot (and not side branches).
Before topping: After topping: