Please tell me what the coach actually does though. What tactics exist in Baseball or am I being stupid.
There aren't many tactics in baseball. Substitution rules are the same as football, once a player leaves the game he cannot return, so the biggest in-game decisions Managers have to make in baseball are pitching changes. These are often determined by things such as matchups (for instance, it's typically harder to hit against a pitcher who throws from the same side from which you hit, ie - right hander v right hander), pitch count (pitching taxes the arm greatly and over the past 20 years or so tracking the number of pitches one makes in a game has become highly valued, 100 pitches is about standard for a starting pitcher), or of course performance. There are two different leagues in Major League Baseball, the American and the National. I won't bore you with the details, but the National League (the Red Sox play in the American League) is generally assumed to be the more "tactical" of the two as there are different rules in the two leagues.
The truth of the matter is that Managers in baseball are far less important than other sports (this is borne out by their salaries). Their most important role IMO is man management as the baseball season is extremely grueling (162 games over 6 months) and players go through massive ups and downs. Very few Managers are viewed as innovative in any tactical way, though. The only one in the game today who's really seen as such is the Tampa Bay Rays' manager, Joe Maddon. He was the other finalist for the Red Sox job ten years ago when they hired Francona, so they had a really good eye then (and neither man at the time was proven).