Why was the Federal Calvary vital on the first day of gettysburg?

Answer 1

On the first day of the Battle of Gettysburg, Union cavalry forces played a vital part in holding back the advancing Confederates. The first to spot and to resist the east-moving Rebels, Union cavalry dismounted and used their carbine rifles in an efficient defense of the more numerous enemy. While they were eventually routed from the field on that first day, these Union troops bought time for the rest of the army of which they were a part and thereby contributed to the victory that arrived two days later.

Answer 2

Calvary attacks prevented the South from gaining the high ground.