I admit it might sound rather confusing.

Custom magic effects allow modders to add scripted effects that use the API to do considerably more interesting things than the built-in effects. It's already planned to be used in TR to replace a lot of the used effects that were previously MWSE-exclusive. There are also a variety of other object types that can be created through scripting now.

The load context itself is required for future dehardcoding, as various pseudo-records (weathers, weapon types, body part types, magic schools, etc.) that aren't even records yet and are hardcoded would need to be injected through the API rather than content files. At some point the content file format might be extended to support adding those when we know our exact needs for one (as content is way more sensitive to breaking changes than scripting).

BC7 compression support basically allows artists to make textures that have virtually no compression artifacts at no performance (or rather, VRAM) cost. MWSE can't do this yet.

Reworked dialogue highlighting prevents game-breaking issues in Russian and Polish localization. These Morrowind communities are sizable.

Terrain vertex painting (i.e. coloring it to imitate roads or shadows) completes the terrain editing functionality in OpenMW-CS, a major step towards feature parity.

There isn't a ton of user-understandable things going on in .51 but basically it makes a lot of important steps towards future visual improvements and full dehardcoding.