Woodworking requires glue and a good glue joint is one that has just the right quantity of glue spread evenly over the entire surface being glued. When you run the glue bottle down the edge of a board and simply push up another board, the glue tends to spread out as in the first photo, where I am pushing a piece of Plexiglas over the line of glue. It is easy to end up with parts of wood that get no glue. That is why we apply a bead of glue to the edge of a board, bring in the mating board and then rub them together to spread the glue out for an even and complete coating.
Another way to spread out glue, either on the edges of boards or over larger surfaces is to use special glue spreaders. The one in the second photo shows how the glue is applied to the roller and the roller spreads it out on the surface. Good woodworking glue bottles actually have several gluing heads for special uses, such as heads that go into dowel holes and apply the glue to the sides of the hole all around, or into the slots of biscuit joints, applying the glue to both sides rather to the bottom of the slot.