If you have spent serious amounts of money on professional camera gear, you are going to be wanting to protect that gear from all kinds of possible damage. One thing people tend to forget about is how moisture can affect your gear if left to build up.
Now making sure your camera is kept safe from water, moisture etc, should be common sense. But I now of several people who, after being caught in a rain shower, put their camera straight back into their camera bags and continue about their day.
Leaving your camera in the bag with water droplets etc on it is a bad idea. Water vapour has to escape somehow and being locked in the bag is just going to cause problems. If left, you run the risk of causing rust to form on metal elements, water getting into battery compartments, condensation forming in your lens' and also in your camera. The last place you want any water or vapour is in your camera. Think of the damage it can do, just to the sensor itself.
Apart from making sure your gear is kept dry and away from the elements, there is one item I recommend every photographer should have in their camera bags to prevent these problems occurring if water or condensation is present.
SILICA GEL! A little bag of silica gel balls in your camera bag, is enough to keep it moisture free and your gear safe. These little bags soak up any moisture lurking in there and remove the risk of it causing any problems.
I have never paid for a packet of these, all the ones I have in my own camera bags have been kept over from when I find them in any new item I have brought. They are pretty much everywhere. Just keep a look out. If you can't find any, you can buy them from the internet and they cost next to nothing. So for the sake of a few pounds, It is totally worth investing in some for your camera bag.