Mixed melee/ranged units question

Hey guys,

I’ve noticed that a lot of recent lists take veteran squads with a 50/50 split of thunder hammers and infernace heavy bolters/frag cannons. What is the reasoning behind this?

I would have thought it would be much better to have units dedicated to either ranged or melee. it seems to me that the thunder hammers would be useless until the unit gets closer, and then the heavy weapons become useless once melee begins. surely im missing something here?