I haven't done any competitive battling for a while, but when I played VGC the logic went like this:
Stronger but riskier moves are better for frail yet powerful Pokémon like Mega Lucario, Hydreigon, Terrakion and Mega Lopunny. Yes, Mega Lucario does become less durable the more you use Close Combat, but it's already not the strongest Pokémon and it is much better to OHKO a threat like Mega Kangaskhan and suffer the stat drop, then to use a weaker attack like Sky Uppercut or Aura Sphere and risk getting KOd first. The same logic applies to moves like Draco Meteor, Fire Blast, High Jump Kick, Stone Edge, etc.
TL;DR The best defence is a good offence, as they say.
Safer but weaker moves are better for bulkier Pokémon, if you're playing the long game, or if they have secondary effects that are more beneficial for you than a simple KO. A good example is Rain teams. All Rain teams ran a Politoed and at least one other Water Pokémon. Depending on what build your Politoed had, you could choose Hydro Pump (mostly Scarf Politoeds) for that sick Rain boost, or you could run Scald (mostly Sitrus Politoeds) for the chance to inflict a burn. Other popular Rain Pokémon like Kingdra and Ludicolo had to choose as well. Using weaker moves is often a tactical decision, and they are better for Pokémon that don't dish out much damage anyways. A 2HKO on Mega Kangaskhan with Hydro Pump isn't as good for you as a 3HKO and chance to burn with Scald. Because weather lasts for a limited time, landing a slightly weaker boosted attacks can better than totally whiffing a more powerful one — hence why a good portion of Mega Charizard Y ran Flamethrower over Fire Blast.
As for my other point, about playing the long game, we look to more stall-y Pokémon like Suicune and Mega Venusaur. Sure, Hydro Pump is stronger, but Suicune benefits more from the burn chance and higher PP because it's planning to stay on the field for a long time. Mega Venusaur could use Petal Dance or Energy Ball to attack, but it's usually better to use Giga Drain to help it stay alive for longer.
TL;DR Pokémon that focus more on waiting out the opponent rather than beating them to a pulp benefit more from weaker, more tactical moves.
There are some exceptions where you want to have the stronger but riskier attacks on slow/bulky Pokémon. A prime example from my VGC days was Ferrothorn. Ferrothorn only has two decent Grass STAB attacks — Seed Bomb and Power Whip. And believe me, it would be a cold day in hell before you saw any Ferrothorn with Seed Bomb. Although my previous argument about bulky Pokémon would point to Power Whip being the less favourable move, this was not the case. With a bit of Attack investment, Ferrothorn could become a real threat to the omnipresent Water-types like Rotom-Wash, Politoed and Suicune. It especially needed to pose a threat to and remove those Pokémon before they could land a Burn, which would cripple Ferro's utility. It is much harder for Ferrothorn to apply pressure to these Pokémon with Seed Bomb than it is with Power Whip, despite the fact that Power Whip might not hit the target.
Alternatively, strong "sweeper" Pokémon might opt to use a weaker move in the interest of self-preservation or to gain the element of surprise. I have two examples: First and foremost, the difference between Return Mega Kangaskhan and Double-Edge Mega Kangaskhan. While Double-Edge is undoubtedly the stronger move, a lot of players chose Return instead to avoid the nasty recoil. Second is Landorus-Therian, easily one of the top Pokémon any year he is allowed in the format. Near the end of the 2016 season some player began using Focus Sash or Choice Specs Landorus, which changed many things about using Landorus, the most notable being dropping the stronger Earthquake for Earth Power. This was a situational tactic used to surprise an opponent who expects a physical Landorus by KOing an unsuspecting target, or simply to keep the typing and coverage of Landorus while avoiding Earthquake's damage to your teammates.
TL;DR Sometimes bulky Pokémon need a riskier move to KO other bulky Pokémon, or a weaker move may be better than a stronger one if it fits your strategy/situation better.
I certainly hope that this helped and made sense! Good luck teambuilding!
