What did John Buford do to make the Battle of Gettysburg take place?
Brig. General John Buford was a brilliant man and a hardened veteran. Upon arriving in Gettysburg on June 30, 1863 he and his men spied the advance column of confederate troops under the command of Johnston Pettigrew. He rode through the town, up the Emmitsburg Road, and out Washing Street. When he reached what is now called Seminary Ridge, he realized the importance of the surround area. He vied to hold it, at all costs. He set up advance skirmish lines, under Col. William Gamble on distant Herr Ridge. These men were in charge of alerting the main line of a confederate advance. The main defensive line itself was placed upon what is now called McPherson's Ridge, although then it did not have such a name. In the early morning hours of July 1st, 1863 as the confederates under Brig. Gen. Henry Heth marched down the Chambersburg Pike, a single shot rang out. Lt. Marcellus E. Jones of the 8th Illinois Cavalry had borrowed a carbine from Levi S. Schaffer and fired a shot, the Battle of Gettysburg had begun. All in all, what John Buford did to make the Battle of Gettysburg occur when it did, was the fact that he decided to hold the ground at all costs.
A very nice answer, and I commend you for even knowing about Herr Ridge, a name omitted in many history books. Just to add to what you said, Buford also sent word to Reynolds to come up quickly to support him, which Reynolds did. So, it was Buford's delaying action plus his quick message to Reynolds that made the clash between the Army of Northern Virginia and the Army of the Potomac happen at Gettysburg.
Great detail but I believe that Buford's important service was to enable the Union Army gain the important high ground. Reynolds was the first Corps commander to arrive and he relieved Buford and his troopers. Of course Reynolds was killed almost immediately by a Confederate sniper. The hills were starting on the left flank Little and Big Round Top, Cemetery ridge and the bloody Culp's Hill where we all know that the son of the owner of the Culp Farm son was with the Confederates at Culp's Hill and was killed on his own land.