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How does a blowpipe help a glassblower form a vessel?

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fish of core-made glass — glassware Fish of core-made glass with “combed” decoration, Egyptian, New Kingdom, 18th dynasty (c. 1363–46 bc). In the British Museum. 0.141 m × .069 m.

The blowpipe is a hollow metal tube essential to the craft of glassblowing. Contrary to the laboratory instrument used for heating, the glassblower’s blowpipe is used to gather, inflate, and shape molten glass into a vessel.1

The process functions as follows:

Gathering and Inflation: The glassblower collects a mass of molten glass—often described as having the consistency of molasses—onto the end of the hollow pipe. By blowing air through the tube, the artist creates a bubble within the molten material.1
Shaping: Once inflated, the glass is formed into a vessel through a combination of blowing, swinging, and rolling the pipe on a smooth surface known as a marver.
Symmetry and Manipulation: To maintain the symmetry of the piece, the pipe is often trundled on the arms of a specialized bench called a "chair." While the glass remains soft, the artist can further manipulate the form using hand tools, shears, or by attaching additional elements like handles and feet.1

This technique allows for both free blowing, which relies on the artist's skill, and mold blowing, where the pipe is used to inflate the glass into a pre-shaped form.1