If you have no creature tokens to copy, then populate fizzles
You make the choice to have the creature with unleash enter the battlefield with a +1/+1 counter or not as it's entering the battlefield
This means you can sacrifice a blocked creature, you can let damage resolve and then finish off an opposing blocker post-combat, or you can do something filthy like get in a bunch of damage with a Hellrider and then sacrifice the Hellrider itself for six more damage
Grolnok is everything you could want from a self-mill Commander in these colors: affordably costed, open-ended in deckbuilding requirements and templated to not lose its accrued value if removed
In this tournament, adults and kids (not necessarily parent-child teams, though I imagine it was highly encouraged) duked it out in a 2v2 tag team format, with the top 8 teams receiving both this card and a special Kangaskhan dorayaki that probably tasted like victory or so the story goes
There are offensive, defensive, and value reasons to want to use these effects