IO540, for example, there is a slow aircraft on holding point waiting for IFR departure and a 737 is just to be established on the ILS. If the slow aircraft took off following IFR departure and the 737 would for some reason have to execute a missed approach, it could lead to loss of separation (due to large difference in speeds). Usually, the slow traffic on the ground has to wait long enough so both the approach and missed approach/departure area is clear for long enough so they can depart without messing with other traffic. But sometimes ATC ask the fast aircraft on approach, if they accept visual traffic pattern in case of missed approach, and if they do, the ATC issues something like this: "xxxxx, in case of missed approach, continue visually with left traffic pattern runway xy, climb to 3000ft".
As said, in VMC one can do a lot of things visually (descent, climb, traffic/terrain avoidance, approach, departure, missed approach), but still remain IFR for the entire time.