The Founder of the Visitation Order, St. Francis de Sales, wrote in depth on the spiritual benefits resulting from a retreat, a time set apart for God.
In his Treatise on the Love of God, St. Francis de Sales wrote: ” The motive of divine charity pours forth an influence of particular perfection on the virtuous actions of those who have especially dedicated themselves to serve God forever.
Such are all those who of set purpose make deep, strong resolutions to follow God’s will and for this end make retreats of several days, so that by various spiritual exercises they may arouse their souls to a complete reformation of their lives. This is a holy method, familiar to the early Christians.
I know that some do not think that such general self-oblation extends its power and carries its influence into the actions we perform afterwards, except insofar as in their exercise we make particular application of the motive by dedicating them in a special manner to God’s glory. However, if I have resolved within my heart to give a hundred crowns out of love of God, even though I afterwards distribute them in my spare time with a distracted mind and without attending to my resolution, yet the entire distribution does not cease to be made through love. A man who had deliberatedly made himself a servant of God’s goodness has thereby dedicated all his actions to God.
Acting on this truth, everyone should once in his life make a good retreat, therein to cleanse his soul from all sin, and then to make a deep, solid resolution to live entirely to God. “
Source: Treatise on the Love of God, Book 12, Chapter 8, St Francis de Sales