Why store table relations in a different table?

I hav开发者_运维技巧e seen people often use a separate table to store relations between tables.

Example, Table Company_companys has two columns a nodeid which is a company id and a linkid which is a companyid of a different company.

Why do people do this when the same thing could be accomplished with an extra column in the Company table? Is is for performance reasons? or is there some other reason?

EDIT: Thanks everyone for the answers and examples, I understand now that in order to achieve First Normal Form when a many to many relationship is necessary, a separate table is needed to store the multiple links. When thinking about the above question I had pretty much forgotten the concept of many to many relationships so I was thinking about it from a one to many relationship point of view :)

It's probably because it's representing a MANY TO MANY relationship. With the approach you are mentioning, you only have a ONE TO MANY relationship.

So, in your example, if your tables semantic mean to represent any company probably relating to many other companies, you need a separate table for that.

The additional "link" table is required if you have Many to many relationship

It is to resolve an issue with many-to-many (..) relationships. Using a third table with foreign keys creates 2 one-to-many relationships as a many-to-many relationship is usually bad database design.