Well, good question. Friar John does not explain it. He only says "Going to find a barefoot brother out, one of our order, to accompany me" without explanation. However, we can guess why he did not want to make the long journey to Mantua alone: it would be safer and less tedious. It's unfortunate that he picked as a companion someone who was suspected of being infected with an "infectious pestilence."
Friar Laurence married off Romeo and Juliet.
Friar Laurence is the friar who marries Romeo and Juliet.
Friar Lawrence married Romeo and Juliet in his cell.
Juliet, Romeo, Tybalt, Nurse, Benvolio Or . . . Juliet, Romeo, Friar Lawrence, Nurse, Capulet Or . . . Juliet, Romeo, Friar Lawrence, Capulet, Tybalt Or . . . Juliet, Romeo, Capulet, Mercutio, Tybalt Or basically Romeo and Juliet and any three of Nurse, Friar, Capulet, Mercutio, or Tybalt.
Romeo, Juliet, and Friar Lawrence are in Friar Lawrence's cell.
Friar Lawrence wants Romeo to do different things depending on what part of the play you are talking about. When they first meet, the friar tells Romeo to go slowly in his relationship with Juliet (as if!).
Friar Lawrence was the priest in Romeo and Juliet
The friar.
Friar Lawence
It was both Romeo's and Juliet's idea to marry eachother.
Romeo asks friar Laurence to marry him and Juliet in secret
hope from Juliet, romeo and the friar that romeo and Juliet will live together in love for the rest of their lives.