When you check-in on arrival, that can cause two problems; one, people (whom you don't want to meet) can come to the location to find/meet/stalk/harass you; and two, people know that you're there and not somewhere else (so they know your car/house/office/dog is potentially unguarded).

You can use checkins (especially those of Instagram or Foursquare addicts) to determine someone's pattern; if they always go to Starbucks in the morning for a snack, then go for a run for 30 minutes, then hit the grocery store on the way back, you can figure you probably have 45 minutes to loot their house, search their stuff, etc. before they get back. If you've seen the inside of their house before, you might know that there are spare keys, or a rarely-used door that you can unlock, etc. which gives you more access to the house later, even if your timeframe is limited.

Etc. etc.

Anyway, checking in as you leave throws off that timetable. People can't come meet you (because you're already gone), and you won't tell people you're out of the house until later, cutting down on their window of opportunity.

I understand the problems with checking-in as you go places. What I don't understand is the value in checking in as you leave. Why publish a check-in at all? What do users gain by checking-in?