Another benefit of CC is hiding your expensive units in combat. A shooty unit can be shot by anything, but reach combat and you can be more selective over who hits back. Shooting units can certainly use line of sight blocking terrain to limit what comes their way, but if there's enough terrain then the balance between shooting and combat is already being balanced.
I understand there was a high level (?) US tournament recently where the top three armies were daemons, daemons then tyranids. IIRC Tau (despite their codex) didn't fare as well as expected because the tables were well covered with terrain
Just found the article. Tau's highest ranking was 10th
The enemy can't hurt you back when you're shooting at them, and you can disable portions of their army in the crucial first few turns. Assault takes time to initiate, which gives your opponent options in howe he redeploys to deal with it. Shooting punishes deployment mistakes, removes the deadliest units without engaging them, and is generally superior.
You can win games through shooting, but assault? Not so much. You will almost always come up against someone who's better at you in assault unless you're a maxed out unit of Wraiths or Paladins with attached Draigo and Librarian or something silly like that. If you come across Draigowing with your close combat army, you're in trouble, because you have to deal with them when they're strongest and they remove your strengths through Grenades. If you're a shooting army, you can generally deal with them in short order. The reverse is not true for the opposite scenario, however; you can't deal with a powerful shooting unit through combat just because it's combat (of course, you can technically deal with anything in combat if you're packing 30 Deathwing Knights or something, but I'm talking efficiency/practicality here) - your best chance of taking down a Riptide is not through charging it, because it's tough, could beat you, and it'll kill what you throw at it while running away like the giant cowardly Tau it is. You have to shoot it, or throw very expendable units at it to force it into either poor target priority or combat.
I would have thought the most efficient way of dealing with a riptide is getting something with an ID causing weapon into combat with it. Easier said than done, but with shooting you'd have to hammer away for a long time with multiple units. Maybe I'm looking at it too much from my nid perspective, but for me I'd try to charge it with my flyrant asap. That's quite a specific case because my hive tyrant is kitted out unusually (wings, 2x scything talons, toxin sacs, implant attack) but you could do the same thing with a bloodthirster too. Or a great unclean one if you can catch it. ID aside pretty much any CC monstrous creature could hack it to death without shooting support (avatar, keeper of secrets, daemon prince, wraithknight, trygon, CC tervigon etc)
Other good ways would involve massed poison or rending attacks