What is the rationale behind the provision of diplomatic immunity?

Question

What is the rationale behind the provision of diplomatic immunity?

Answer 1

We extend it to foreign diplomats so they extend it to ours.  We would not want our diplomats in countries with shaky civil liberties records arrested and thrown in jail (or worse) under the foreign system.

Answer 2

I think the idea was that diplomatic personnel have to feel free to come and go without worrying about getting arrested.  Countries can use fake criminal charges to make diplomats hostages.  It expresses a level of trust between countries that have diplomatic relations.

Of course like every privilege, it is sometimes abused.