Why use an int as a lookup table's primary key?

I want to know why I should use an int as a lookup table's primary key instead of just using the lookup value as the primary key (which in most cases would be a string). I understand that using a nvarchar(50) rather than an int would use way more space if it is linked to a table with many records. But on the other hand using the lookup value directly would basically save us doing a join. I can imagine this would be a big saving if the join is always required.

What are the advantages of using a int primary key (specifically for a lookup table) other than it being "the standard thing to do"?