Stefan
Teams use to go with fewer pitchers than they do today. Teams would go with 3 or 4 starting pitchers. In the early 1900's they would go with 2 or 3. Also pitchers use to pitch many more complete games than today relief pitchers were not used much until the 1970's.
Several reasons for the change in the way starting pitchers are used exist. After the 1968 season the pitching mound was lowered by several inches, 6 I think, because most games were low scoring with teams only scoring 1 or 2 runs a game. In 1973 the american league started using the designated hitter this was to allow for more runs to be scored because the pitcher would not bat. The biggest reason though I think occured in 1975 or 1976, don't remeber which, when Andy messersmith was declared a free agent. Until that time when a player signed with a team he was basically forced to play for that team his whole career. Once players could move from team to team the amount they were paid increased greatly and owners felt like they couldn't risk hurting the pitchers. Also expansion has hurt the quality of todays pitchers to many teams chasing to few good pitchers. With the large amount of money todays players get they no longer need to get other jobs in the offseason, they can train year round. Todays hitters are much stronger than they use to be. Also pithers today are not allowed to throw the ball inside, Bob Gibson was one of the best pitchers in the 1960's but today I don't think he would be nearly as good beacuse he would be thrown out of many games for pitching inside.
If you look at the team today with the best pitching staff, the Braves, you will learn that they are one of the only staffs that throw between starts. Many baseball people will tell you that todays pitchers are not as good as they could be because they do not throw enough. Most teams limit the number of pitches that a pitcher can throw in a game to around 100. In the past it was not uncommon for pitchers to throw 150 in a game.
Today every pitch is recorded and every hitter knows that in this situation this pitcher like to throw a certain pitch. If you know what pitch is going to be thrown it make the job of hitting it much easier. Tony Gwynn one of the best hitters the last 15 years watches several hours of video tape every day of the pitcher he will face.