The idea is to support customizing the source query for a specific set at the model level, e.g.:

modelBuilder.Entity<Product>()
    .FromSql("SELECT Id, UPPER(Name) AS Name FROM Products");

Or using LINQ and some extra state such as the DbContext

modelBuilder.Entity<Product>()
    .FromQuery<MyDbContext>((source, context) => 
        source.Where(e => EF.Property(e, "TentantId") == context.TenantId);

One possible advantage of having this in the model is that we would take it into account even if the entity type is included in the query as part of a navigation, e.g. it would make it possible to add additional filters on navigation properties even if they are populated using Include().