Generalizations

I hope it is ok to ask this here.
Im having trouble with ERD's.
I have been given a definition on which to draw an ERD. I did so, and began transforming the entities and relationships to relations using the following steps

Step 1) Transform entities
Step 2) Transform weak entities
Step 3) Transform Relationships
Step 4) Transform Generalizations
Step 5) Simplify.

When I came upon step 4 I realised I had two opportunities to generalize. A Doctor and a Patient (Both with SSN as their primary keys) could both be specializations of a 'People' entity.
Likewise Pharmacy and Pharmaceutical company are both specializations of a 'company' entity.
I dont understand why we have the generalizations. It seems to me that we are just creating a relation for the sake of it.

Could someone put me onto the right track? TIA

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