Because paying down the highest-interest debt first minimizes the balance that accrues interest at the highest rate, for the same total amount paid. This is a mathematical property of compound interest, not an opinion.
Behavioral research on debt payoff (notably studies referenced by personal finance educators like Dave Ramsey) suggests that quick wins — fully eliminating a small debt — can keep people motivated to stick with a payoff plan. If Avalanche's extra interest savings are small for your situation, Snowball's motivational boost may be worth it.
Group smaller debts together, or run the calculator for your 3 largest/highest-priority debts first, then repeat for the rest once those are cleared.