What Depth Strider changes
Water normally slows movement significantly and reduces jump height. Depth Strider reduces that drag per level, so at level III you move through water at close to your land speed. It affects horizontal movement and lets you jump in water more normally.
Depth Strider vs Frost Walker
The two enchantments conflict on boots. Depth Strider keeps you fast in existing water; Frost Walker freezes water into ice as you walk. Choose by terrain: Depth Strider for oceans, rivers and the nether's lava oceans (with fire resistance), Frost Walker for crossing frozen or dangerous liquid surfaces at speed.
Where it earns its place
Ocean travel, ocean monument raids, drowned farming and any build near water benefit. It also helps when mining under water and when crossing the nether's magma field corners. Pair it with Respiration and Aqua Affinity on the helmet for a complete underwater kit.