Pros and cons of creating a table with key

Hi Folks

I have some tables where there is no key defined in the source system. What is the best way to build such tables. In my case the source system is Teradata.

I have 25-30 columns in dimension and what is the best way to load this data where there is no.

What will be the Pros and Cons if I have a table without a key  or if I create a table with 25-30 keys.

regards

Poonam