Natural teeth shift in your mouth if they don't have something to stop them. That is why you may have noticed that after you lost one tooth in your mouth, others started to become loose.
Partial dentures close gaps in your mouth and also give your opposing teeth something to chew with. A partial denture uses clasps to prevent the denture from lifting until you want them to come out. Only a properly designed partial denture will not cause harm to your existing teeth. Poorly fitting partial dentures can put too much pressure on your natural teeth and cause them to become loose.