This is carried out to increase the clearance between the ground level and the lower branches either to allow access below the tree, to clear sight lines, improve views or allow light to penetrate to the ground. This can be a simple yet very effective solution.
BS3998 (2010) recommends;
“Crown lifting that involves cutting back branches to the stem(s) should preferably not result in the removal of more than 15% of the live crown height. Also, the remaining live crown should make up
at least two-thirds of the height of the tree. Unless the objectives change, there should be no further crown lifting”