Gaffer's tape as the name implies is used by the gaffers in the film industry. It is a multipurpose tape used for just about everything on a movie set. The cameraman and the focus pullers also use them to label their film and mark the lenses so that they know certain prefocused distances for certain shots. For those who don't know, in the film industry, focus is determined by measuring the distance from the camera to the subject and the focus is set according to the distance markers on the lens barrel. That's why a tape measure is a very essential tool for a cameraman.Originally Posted by yumyum
Photographers also find it useful from attaching gels to a light to taping cords down so people don't trip over them. It can also be used to attach filters(the cokin type or the Tiffen Series 9 type filters) to lenses. There are many other uses for gaffer's tape in the studio and on location.
Also for theatre, the coloured tapes are used to mark certain points on a stage so the actor/dancers know their positions so the lighting designers can set their lighting.
As far as helping a DSLR, well, personally I don't find anything to be gained by covering a body or lens, other than asthetic "value".
Over time the tape will leave a residue and you will have one hell of a time trying to remove the gunk of your lens and/or body. Just like plastic, over time it will deteriorate.
Just my $0.02.