Bare root plants are less expensive, easier to handle, and easier to prune into a desired form than canned (potted) plants are. Also, they get established into the garden easier. Yes, even with less roots, they disperse their new roots directly into the surrounding soil more efficiently than secondary roots escaping from crowded roots that had been confined to cans of media (potting soil).
Bare root plants have no incentive to stay confined. They get planted while dormant, and wake up surrounded only by their new soil, with nothing else to get in the way. Canned plants might have been circling their roots within a limited volume of media for a while, trying to find a way out. Once they get in the ground, they may not like what they find there, and try to stay close to the familiar.
Bare root plants should not get too much of a good thing. The need only minimal soil amendment. They might like a bit of organic matter to retain moisture and to keep the soil loose while they get oriented to their new home. A bit of fertilizer would be nice too. Yet, new bare root plants should not get so much amendment that they do not want to disperse their roots beyond the planting hole.
Planting holes do not need to be very big at all. They should be wider than the roots can be spread, but not deeper. Loosing the soil and adding amendments below will cause new plants to sink. Graft unions of grafted plants must remain above grade. Plants with big roots, like fruit trees and roses, prefer their roots to be spread out over a cone of soil in the center of their planting hole.
Most bare root fruit trees are sold with more branches than they need, so should be pruned after planting. Some might get pruned by half. The superfluous stems are there both to cushion the trees in transport, and also to allow more choices for pruning. Some of us want to prune down to lower branches, while others want to prune up to slightly higher branches. Berry canes need very small planting holes, and get pruned back to only two or three buds above ground.