Just as these mediums enable "signals" to be received, and translated, into a manner that can be appreciated by our normal five senses so too, what are often called "spirit mediums", do the same. Using what is often termed the ability called "mediumship".
The "signals" in this case are signals being directed from spirit people/sources. If the medium hears these signals then they are often called clairaudient. If seeing them clairvoyant. If sensing them then clairsentient. This information is then passed on to people in the physical world via one, or both, forms of mediumship.
If physical mediumship interests you then I suggest you take a trip to this Physical mediumship page.
Why have mediums/mediumship ?
There are many answers to this question but perhaps two of the most common are ;
Mediums who contact dead relatives/friends of people who are still physically alive are often a great help to these people (who are often unsure as to whether there is life following one's physical death). They are often helped to know that their loved ones are still "alive", albeit in a different dimension.
Mediums who contact their own spirit guides, and/or the guides of others, often find that conscious contact with them helps them/others in achieving their spiritual objectives.