It is unusual to have a social security hearing without a vocational expert present to offer testimony. Very often, the vocational expert makes or breaks your social security claim.
The vocational expert, hired by Social Security, assists the Administrative Law Judge by analyzing your residual functional capacity (advising the Judge what activities you are able to do despite your disabilities and limitations) and determines whether jobs exist in the national economy that you can still perform given your limitations.
The Judge will ask the vocational expert a hypothetical question incorporating your physical and mental limitations and ask the vocational expert whether and how many jobs exist that you can still perform. If the vocational expert can identify jobs within your residual functional capacity, most often you will be found to be “not disabled” because there is work available, even if you can’t find it.