Почему Entity Framework имеет AddAsync?

Из документация:

This method is async only to allow special value generators, such as the one used by 'Microsoft.EntityFrameworkCore.Metadata.SqlServerValueGenerationStrategy.SequenceHiLo', to access the database asynchronously. For all other cases the non async method should be used.

SqlServerValueGenerationStrategy.SequenceHiLo:

A sequence-based hi-lo pattern where blocks of IDs are allocated from the server and used client-side for generating keys.

См. Также Что такое алгоритм Hi / Lo?

Begins tracking the given entity, and any other reachable entities that are not already being tracked, in the Added state such that they will be inserted into the database when SaveChanges() is called.

https://docs.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.dbcontext.addasync?view=efcore-2.1