I'm not an expert on JBPs or pines in general, but, as far as I am aware, this is done for 2 reasons: 1) aesthetics, it makes the tree look more miniature and in-scale by having less foliage. 2) it allows more light through the canopy through to the lower branches which increases the health of those lower branches increasing their vigor 

If any experts can correct me that would be great!

Like u/SandwichT said, it's partly to allow light to get through to other parts of the tree especially to lower branches which can easily get shaded out by strong apical growth but also to allow light to reach interior parts of branches which can help with back budding. The biggest reason is to help balance the look of the foliage and strength of the tree.

By this time of year the trees look a little scraggly. The foliage from the previous year may be a different length and the apex will have thicker needle coverage then some of the lower branches. We want to rebalance where the tree is putting energy by plucking the tree down to less needles at the apex and leaving more on the lower branches so the tree delivers more energy to the lower branches. This helps to create more uniform needle size and prevents the apical branches from thickening faster than the lower branches.

Plucking each growth tip to 8 to 10 sets of needles makes the tree look more uniform overall. When one growth tip

How come? Can you explain how this improves the tree? Thanks!

Here is a before and after shot of one branch I was working on. 

Generally needle plucking is a January task but it's hard to work on the trees for the Lynden Collection at that time of year. 

The top of the tree and vigorous branches had all of last year's needles removed and then this year's growth reduced to eight fascicles. The weaker branches and lower parts of the tree I left ten fascicles. On a black pine fascicles are two needles held together with a sheath. 

You pull the needles off with your fingers or a tweezers and leave the sheath behind.

Needle plucking day for the Japanese black pines