Fire polishing the ends of glass tubing and rod will seal small cracks in the end wall, reducing the chance of fractures originating from this source.
Fire polishing fused glass is basically "glossing up" any dull surfaces. These might be the result of glass that has had the edges ground, sand blasted or etched surfaces, etc. Fire polishing ends will also reduce cuts and abrasions to you or anything (corks, latex tubing, etc.) coming in contact with the glass end.