One simple situation in which you would like to switch modes on the transponder (ok, so this is not technically shutting the transponder down) is when - for example - a loss of cabin pressure occurs. Since most (all?) commercial arliners will have to perform an emergency decent to somewhere around 10,000 feet it is dangerous for transponders to operate in TA/RA (Traffic Advisory/Resolution Advisory) mode and give climb or descend instructions to avoid other traffic. In this case pilots switch to a "squawk only" setting.