Belcher is a fast combo deck aiming to assemble Goblin Charbelcher or Sea Gate Restoration through cheap blue cantrips and countermagic, so Mystical Dispute is excellent since it counters their many blue spells like Whir of Invention and Counterspell for just {U}. The extra Spell Snare and Consign to Memory add more cheap interaction against their two-mana spells and colorless artifact spells like Goblin Charbelcher and Lotus Bloom. Lightning Bolt and Prismatic Ending are weak since they rarely have relevant permanents to kill early, and Territorial Kavu is a slow clock against a deck that can kill from nowhere, so we prioritize disruption over more bodies.