Thank you very much for your instructive discussion over my questions.
Let me sum up your opinions and confirm my understanding so far:
(1) There are many staff members in this company.
(2) This company has many staff members.
These two sentences are interchangeable.
(my understanding)
Whichever language it may be, whether a sentence sounds natural or not generally depends on the context.
Even though different sentences can describe the same fact, there should be some slight difference in the idea, feeling, emphasis or focus of the speaker.
a, There are two legs with a man.
b, A man has two legs.
a- makes infinitely more sense than b
(my understanding)
Even to my ears, the sentence (a) sounds odd.
I reflected the reason in myself, uttering these two sentences repeatedly to figure out what difference I can feel between them and why.
When I first compared these two sentences without any context, I unconsciously felt that the speaker is referring to an attribute of a man. In that sense, taking "a man" as the subject of the sentence makes it clear that the speaker focuses on "a man" and is describing how he looks, what he possesses in nature.
If so, even in the examples (1) and (2), the sentence (2) could express its attribute. The speaker is describing the company, while the sentence (1) objectively states that there are a lot of people and that they are in the company as an additional information.
c, There are four eggs on the plate.
d, The plate has four eggs on it.
c-good / more natural than d
d-fine/ sounds odd (though grammatical)
"There is/are" construction can express the number of things/persons with equal naturalness.
However, in a S-V construction, we may feel more comfortable when the S is Personal or something 'alive' than when it is just a thing/things, because a thing does not do anything with its own will.
In addition, "having four eggs" is not the attribute of the plate. Maybe it would sound more natural if I say,
"The plate has four roses printed on it".
Any additional advice is welcome.