Any expert on skimmer can tell me the purpose of this non-syphon tube on the water outlet size of the skimmer. My skimmer is an external, i want to removed the tube completely due to space issue.
It is needed but maybe you can re-route it. I would not cut it off until you are sure you can fabricate a replacement. If the riser is a standard PVC O.D. you can buy PVC elbows, a section of pipe, primer and cement for less than $20. With what you bought, cut and dry-fit the pieces to route them to a place that fits your space such that bottom fitting matches the location of where you want to cut off the riser on the skimmer and the top of your replacement riser matches the height of the one pictured. Then cut of the skimmer riser, place skimmer in final location and again dry fit it all together to ensure fit. If necessary make any adjustments then match-mark your fittings to pipe with a sharpie. Disassemble then finally prime then cement it all in place one piece at a time. If you are not familiar with this type of work hopefully you have a friend that is - I don't want to be the one leading you to ruin an expensive skimmer.