It's about apparent depth-of-field mainly. Field depth, (that is the amount of a shot's 'depth' which is in acceptably, critically if you like, sharp focus), is usually a function of the focal length of the lens used, whether prime lenses, or part of the range of a zoom. Because depth-of-field is shallowest at the more extreme 'telephoto' end of the scale it usually pays to back off a bit and re-frame your shot so as to register it in the way you wished, by using telephoto, instead of going too close. Some camcorders, sadly, allow this only to a limited extent. On the other hand, ultra-close is another way of achieving limited field-depth.
There is a bonus to that, as well, as it is possible to create a powerful '3D' illusion, by isolating something in critically sharp focus, against a background which is just a tad 'off' providing your camcorder has the 'critical-sharpness' firepower. The other way, if you have the control, is to adjust your camcorder's dynamics, in any of a number of ways which will cause the lens's diaphragm to open-up a bit, which also reduces depth-of-field. An easy way, without having to modify your camcorder's settings, is to screw-on a neutral-density filter which accomplishes the same thing. Again, low lighting will bring about the same effects, but insufficient light will tend to will raise noise-levels.
At the end of the day, it's a whole heap of 'trade-offs'. Having always been a fan of carrying a battery of lenses, I have been pleasantly surprised what my camcorder (a 'Panasonic' HDC-SD900), is capable of doing, without 'supplementaries' or filters, just by having a damned good lens, and image processing to make the most of it.
Ian Smith
Dunedin , New Zealand.
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