The big issue is that none of us actually know if we have the virus. Many people have it and are symptom free. Some are symptom free for the first few days then develop a slight illness. If you have the virus, you shed it. This can be via coughs, sneezes, touch or even talking. If you sit on anything, use anything, touch anything etc you will be shedding the virus. All this can be mitigated by washing hands thoroughly with soap and water or sanitiser, wearing disposable gloves and face masks and not touching eyes, nose or mouth.
The problem is that one lapse can pass on the virus. If you cough unexpectedly, sneeze unexpectedly, lean forward and laugh etc.
That’s why the guidance says “stay at home” . By not going into another home or garden, you are minimising your chances of getting or passing on the virus.
It’s the only sure way to reduce the risk and to help slow down the contagion until a vaccine is available.
If you visit or are visited, you take a risk.