Wood grain absorbs glue - different woods and grain cuts more or less. If Titebond is applied to an end grain much or most of it could wick into the end grain leaving little for bonding to the 2nd piece of wood. Double gluing is the application of glue onto bare wood, allowing it to be absorbed (filling the grain) followed by a 2nd application of glue to the "previously glued surface" resulting in a much more consistent and strong bond.

The red line is the glue used for the butt joint.
The purple fillets are of glue and are simply applied / spread with your finger. If you're using Elmer's glue - you can like your fingers - yum!

Now for Zor to tell me that I'm clueless! Won't be the 1st time nor the last ...