Why did he take hdnfilepath??

Question

Idk why the developer had taken the hdnfilepath give me a proper knowledge about it.what is the use of using hiddenfeild in source page of asp.net application. 

 function LoadDocument(url) {
             debugger
             $("#ViewDocumentmodal").modal({ backdrop: 'static', keyboard: false},"show");
             var imgFull = document.getElementById("imgFull");
             var hdnfilepath = document.getElementById('<%= hdnfilepath.ClientID %>'); 
             if (url != "" && url != null && url != "0") {
                 imgFull.src = url;
                 hdnfilepath.value = imgFull.src;
                 imgFull.style.display = "block";
                 return false;
             }
             else {
                 $("#ViewDocumentmodal").modal("hide");
                 alert("Image is not available");
                
             }
         }