Good question. State has 13000 employees. It has missions in 172 countries. That's 75 employees per country. While 75 employees might be reasonable for a major county like Russia, it is overkill for minor countries like Luxembourg, or Bermuda.
In the 21st century, State has two missions that make sense to me. First to gather intelligence. There is a lotta useful stuff we can pick up by just reading the local newspapers. Like who is who in their government, and in the government's opposition. What are the important industries and businesses? And who runs them? How does the general population feel about things? Geography, good maps are important, and they remain useful for years. During WWII North African operations relied on maps made by the US marines fighting the Barbary pirates of old. All this stuff is important, and gathering it and filing it, is legal.
Second is to give aid and succor to US citizens abroad. Lost or stolen passports, arrest by local authorities, kidnappings, and Lord knows what else. As a mid to lower class American, I like to hope that if I get into trouble in a foreign land I can call on the US consul or ambassador for help.
And, in this day and age of air travel and world wide instantaneous communications, heavy duty international negotiations are handled out of Washington DC, not by US ambassadors abroad. In fact president Eisenhower created the National Security Counsel to bypass a State Department that he considered inefficient, and infiltrated by communist agents like Alger Hiss.
So, if we allowed 20 state department employees per country, that yields a headcount of 5120, a helova lot less than the 13000 bodies they have warming chairs today. If we figure each state department bureaucrat costs $100K a year, than laying off 7840 of 'em would save $784 million a year. That's not quite real money in DC speak, but it's still a useful piece of change.