The one teacher I have outside of the Tibetan Vajrayana tradition is Guo Gu, a lay teacher and the former attendant of the late master Sheng Yen. He also happened to be a university professor of mine for a time, and established the Dharma Drum Mountain Chan group in Tallahassee, FL.
Guo Gu recently had this to say:
A word about retreat practice. There are generally five types of ongoing, interrelated practices in the course of a practitioner's life.
1) Individual practice: this refers to formal daily sitting (or other forms of) meditation practice that we do by ourselves. Individual practice is the foundation of our spiritual path.
2) Daily practice: this refers to practicing mindfulness of our actions of body, speech, and mind amidst daily interactions with people and things. Daily practice extends what we have learned from sitting into all aspects of life.
3) Group practice: this is like the Monday nights, when we meet once a week to practice together as a group. Group practice strengthens and sustains our commitments to the path.
4) Retreat practice: this is done on a periodic basis that allows us to practice with fellow practitioners intensely for a long sustained duration of time under the guidance of a retreat teacher. It can last from 1 day to seven days to even three months.
5) Solitary retreat practice: this is for advanced practitioners, where they devote a long time to practice in isolation. Typically, in our tradition this is reserved for someone who has already seeing their self-nature. Usually it lasts anywhere from a few weeks to many years.
Each of these practices target different aspects of our being; each has its own strengths and weaknesses. Different psycho-physiological experiences and issues emerge from each of these five types of practices. Just practicing one type is insufficient for any real progress on the path.
Retreat practice is an excellent way to deepen our practice because it allows practitioners to engage with the meditation method in a sustained, focused way under a protected environment with a teacher. Practitioners would receive guidance on any issue or experience that wouldn't normally arise in daily or individual practices.
"The Sutras, Tantras, and Philosophical Scriptures are great in number. However life is short, and intelligence is limited, so it's hard to cover them completely. You may know a lot, but if you don't put it into practice, it's like dying of thirst on the shore of a great lake. Likewise, it happens that a common corpse is found in the bed of a great scholar." ~ Karma Chagme དྲིན་ཆེན་རྗེ་བཙུན་བླ་མ་རཱ་ག་ཨ་སྱ་མཁྱེན་ནོ།