LazyReferenceField和ReferenceField Mongoengine有什么区别？

Question

在docsLazyReferenceField中：

A really lazy reference to a document. Unlike the ReferenceField it will not be automatically (lazily) dereferenced on access. Instead, access will return a LazyReference class instance, allowing access to pk or manual dereference by using fetch() method.

我知道在性能方面有差异，但我不明白为什么？我应该总是用LazyReferenceField而不是ReferenceField吗？或者有不同的用例？你知道吗