I think it's some local time difference and also to cater to the US market. Having said that, in summer a 4 pm local time start in mainland Europe is totally fine if you have a three-hour limit to get the race in. Personally think the three-hour limit is totally bogus and it should be determined by daylight viability. There was absolutely no reason to hang the chequered flag out at 6 pm a few laps early in Monaco.
However, with the way convection works it's usually a bad idea to have races stretch through the latter parts of the afternoon. Especially in mainland Europe this is when thunderstorms build up and release. Therefore I'll always prefer 2 pm CET as the race start no matter what.