There seems to be an idea in the photographic world that anything slower than 1/60th of a second cannot be hand-held, and you must use a tripod. This is tripe - unless you have some medical condition resulting in uncontrolled shaking spasms, but I think is a carry-over from Box Brownie days.
The reason to use 1/15th is to expand the light range in which you can take shots without flash, such as sunsets for example, or to bring out the background, even when using flash. You know the shots taken at a function where you get someone looking like a startled rabbit in blackness, where if you had used a 1/15th shutter speed you would have got a nice mellow background to soften the picture.
Of course there are a few tricks to hand-holding at the slower shutter speeds. The first is to steady yourself and that can be done easily by leaning against a wall or a pole (preferably not a chrome one attached to a go-go dancer). The second is to hold the camera firmly in both hands, take a breath in and hold it and then gently depress the shutter button. I have even shot at ½ a second by holding the camera firmly pressed down on the back of a chair. Take a few as some will have obvious camera shake, but you will get at least one good one.
Still on the number 15. There is a theoretical f stop which could be called f 15. F stops after all are only a way of measuring the diameter of the aperture inside the lens, to bring it to its simplest terms. As you go through the usual f stops of f 8 to f 11 to f 16, you are actually cutting the light down by one half each time. The f stop scale is also an inverse ration, as the bigger the number, the smaller the diameter. There is a good mathematical reason for this, but just believe me.
If you really want to get technical, for example, f/16 means that the aperture diameter is equal to the focal length of the lens divided by sixteen; that is, if the camera has an 80 mm lens, all the light that reaches the film passes through a virtual disk known as the ‘entrance pupil’ that is 5 mm (80 mm/16) in diameter. The location of this virtual disk inside the lens depends on the optical design. It may simply be the opening of the aperture stop, or may be a magnified image of the aperture stop, formed by elements within the lens.
The f stop scale is a sliding one, allowing for fractional differences in the light allowed through to the film (or the digital sensors). Most old cameras had an aperture scale graduated in full stops but the aperture was continuously variable allowing the photographer to select any intermediate aperture, and thus it would be possible to shoot at f 15.
The continuously variable aperture cameras slowly disappeared, with ‘click-stopped’ aperture became a common feature in the 1960s; the aperture scale was usually marked in full stops, but many lenses had a click between two marks, allowing a gradation of one half of a stop.
On modern cameras, especially when aperture is set on the camera body, f-number is often divided more finely than steps of one stop or half a stop. Steps of one-third stop (1/3 EV) are the most common, since this matches the ISO system of film speeds. Enough technical details! Understand the numbers 15 and 16 and how they affect your final image, and you will expand your photographic abilities.