Cut 1, make 2. Cut 2, make 4..
Bassically makes your plants more bushy than tall. Some people believe they get way higher yield topping, others say you get same or less. It's an extremly controversial subject. From personal research, I suggest you only top if you're limited on space. If you only have 3 feet to grow a plant that grows 4 feet, top your plant, it'll make it wider rather than taller.