Summer pruning has a dwarfing effect meaning you can control the shape and size of your tree (and thus where the fruit grows) without it sending up so many vertical fast growing shoots known as water-sprouts, the next year. Summer pruning tends to dampen water-sprout growth. The growing season is also a good time to prune especially vigorous young trees, or extremely overgrown older trees that may be getting too big for their space or producing fruit out of reach. Pruning in the summer can begin to stop the endless cycle of pruning out hundreds of water-sprouts and suckers every winter.
Often trees that are very overgrown need several years of pruning to gradually come back down to size or be thinned out appropriately. Excessive pruning at any time of the year can result in an abundance of water-sprouts the next year or unduly stress the tree.

A very overgrown tree in which the Quince rootstock has nearly overtaken the pear that was grafted to it.

Quite a few branches have been pruned out. More will need to be removed in successive years. One of the pear grafts can be seen growing nearly horizontal on the right.