When you take your coker down for maintenance, you can no longer crack Vacuum Unit Bottoms(coker feed) into Naphtha, Light and Heavy gasoil. The Naptha is hydrotreated along with the gas oils. After hydrotraeting the Naptha goes to the Reformer and Heavy Gas Oil to the FCC or Hydrocracker. Without cracking the Vacuum Resid, it must go to storage, Asphalt or be sold. When the coker is down, if you change to Lighter Crudes you have less Vaccum residue and more yield of Naptha and Gasoils so you can keep your downstream units operating with internally generated feedstock along with maintaining finsihed product output. A coker outage can last a week or sometimes longer depending on what needs to be done, the duration would detirmine whether switching crude slates is necessary. Cokers are critical to processing WCS and Bitumen, you can't make enough Gasoline, Diesel and Jet without them.