Because you would not want to use the gases at the pressure they are in the bottles !!!!

Therefore they need to be regulated, there will be two gauges on each regulator one for bottle pressure and one reading the pressure coming out.

You should not be using these things unless trained to do so. During training all will be explained. Do not put grease or oil on the threads of the regulators !!!!, just in case you have been playing.

If you have found a way to use this equipment without a regulator - stop now.

Chris

Gauges are not regulators usually there are both items on a gas bottle but not always.
We need to establish what the OP means before leaping to conclusions

He didn't ask why you use a regulator, he asked why there are gauges fitted. There are plenty of regulators in existence that don't have gauges (though I'm not sure I've seen one for oxygen, admittedly).

Refer to HSE Guidance, , Use of Compressed Gases for Welding, flame cutting and allied processes HSG139, page 15

http://www.hse.gov.uk/pubns/priced/hsg139.pdf

Pressure regulators and pressure gauges

Gauges
Para 33
Pressure readings are useful to a gas welder in order to control the process and check the availability of gas supply; pressure gauges may be fitted for these purposes to cylinders and to distribution pipework and manifold supply installations. Gauges are often fitted to pressure regulators – on the upstream side to give an indication of cylinder supply pressure (a ‘contents’ gauge), and on the low pressure side to indicate outlet pressure. Some adjustable regulators have no provision for gauges but have an indication of outlet pressure marked on the regulator housing, and some deliver gas at a fixed outlet pressure (eg for LPG).

Para 34
Pressure gauges complying with British Standard BS EN 56227, ISO 517128 are preferred. Alternatively, gauges complying with BS EN 83729 may be acceptable. These standards incorporate safety features including safe venting if the Bourdon tube fails and releases gas.

There will be more detailed guidance from BCGA:-
CP7:- The safe use of oxy-fuel gas equipment (individual portable or mobile cylinder supply). Revision 7: 2014 (Minimum safety standards for the use, inspection and maintenance of oxy-fuel gas welding, cutting and heating equipment incorporating individual portable or mobile cylinder gas supplies)

Buy a 2stage regulator that is specifically for oxygen. It will have better "pressure change" characteristics over the life of the cylinders that you use it on, and will require less adjustment.

Use it for 5 years only, as that is the recommended life of a clean functioning regulator. Do not repair it if it ends up damaged, as you are taking on responsibilities of the manufacturer - there are no standards for repairing regulators, just for making them. Just chuck it when it's done.
This includes not putting any PTFE tape, grease, oil or any other hydrocarbon on a regulator bullnose or on the gauges themselves (or anywhere else on the regulator for that matter - just a good habit). This includes protective barrier creams -wash your hands first, change the regulator onto a new bottle, then apply more barrier cream.

If nothing else, having a regulator with 2 pressure gauges will tell you when your oxygen cylinder is done and requires changing, but wil also allow (as has been mentioned in previous posts) you to adjust the oxygen pressure for the work to be done, with some accuracy.

Pretty sure it does not need saying, but the absolute safest way to work with any gases is to only use equipment (including regulators, hoses, flashback arresters, etc) that is designed for use with that specific gas. You can get some pretty nasty surprises if you don't - for example the chap who repaired a rubber hose for acetylene by inserting a copper tube and, when it came to throwing it away into a skip, blew the skip up.

PM me if you want trainer recommended.

HTH
Andrew

Well you learn something new every day. I was not aware that you could still get gaugeless regulators, hence my concern in the post. I thought they had effectively been phased out as the ones with gauges are quite cheap and as noted above only have a life of around 5 years. I also have never seen one despite being in industry for over 30 years. Indeed even trying to search on the Internet took quite a bit of effort to find reference to someone offering one, but I did in the end.

I thought I had answered the question "what is the reason" with my answer "one for bottle pressure and one reading the pressure coming out." I didn't think I needed to go further and explain why knowing the bottle pressure / contents and working pressure were important. I thought it would have been self evident. You would not want to go running out of gas half way through some work or not setting your working pressure and being able to see at a glance if that pressure was dropping (With issues that come with that).

So I apologise Fitzy3 for my concern and tone as it was based on the above belief and thought that no gauges therefore meant no regulator.

I stand corrected

Chris