For most lighting applications, there is significant light loss due to dispersion; as the light from the fixture gets spread across a larger area as distance increases.
An example of a light source where there is very little dispersion loss is a laser pointer, or lasers in general. Because the light beams from a laser are collimated (parallel to each other), there is very little dispersion and light loss due to distance. This enables the laser's dot to be brightly visible from great distance, even though the wattage of a laser (an everyday handheld pointer may have a power rating of only ~ 0.02 watts) can be tiny compared to the distance projected.
Applying this concept to planted tank lighting - good reflections/lens that create light beams that shine directly down (parallel to tank sides) are less affected by dispersion.
Learning from Sunlight
Sunlight is nearly parallel when it hits earth; there is no loss of light due to distance. If you measure light levels 3 feet apart in height you will barely see any light loss.
In common LED light bars that come with "120 degree" lenses, there is significant light loss due to dispersion. Substrate light levels drop quickly as these fixtures are raised above the water line as a lot of light will be cast outside of the tank. Due to the dispersion pattern, surface area near the fixture receives much more light than substrate areas at tank sides. Though common, these type of fixtures give the worst kind of spread in a planted tank.
How to reduce spread
A narrow beam fixture (i.e. 45 degrees lens LED light bar) hung high above the tank, gives a much more even spread within the tank environment compared to a wide angled fixture sitting tank top. If the lens quality is good, there is little light loss due to dispersion even though the fixture is hung high.
The downside of such fixtures is that it requires a setup that allows the light system to be hung high. Also, aside from custom manufacturers, there are fewer fixtures on the market that come with narrow lens angles by default.
Another approach is to use a distributed light array. Multiple T5 tubes with good reflectors and/or a wide LED array gives much more even distribution of light in a tank. The downside is that such fixtures can take up a lot of space, or be bulky.
When does it matter?
It matters when you are picky about plant growth. Distributed spread reduces self-shading in plants; leaves lower down the stem will get incident light rays from distributed fixtures. This means plants in general will be better lit.
Whereas when point source lighting is used; top leaves receives a lot of light, but leaves at lower layers will be deeply shaded. Aquascapers who are extremely picky about plant growth quality will prefer light systems that have better spread.
If you have a vertical hardscape, It matters as well. If you have complex hardscape that occupies vertical space, point source lighting may be obstructed by hardscape and plants beside hardscape will be in shade.
With distributed light sources, more areas can be reached. If you favour complex hardscape, fixtures with good spread are essential.