Why was the 25 amendment necessary

The 25th Amendment deals with vacancy in the offices of president and vicepresident. So, it was necessary to pass it to guarantee a constitutional succession in case a president could not discharge his duties and powers. It was first applied in 1973, following Spiro Agnew's resignation as vicepresident. President Richard Nixon nominated Gerald Ford. Then, it was invocated again a year later; following Nixon's resignation, Gerald Ford was sworn in as president. He later nominated Nelson Rockefeller as vicepresident by using his powers given to him by the 25th Amendment.