You’ll see quite a few posters here with ‘SFO’ or ‘OCDS’ after their name - means they are Secular Franciscans or Lay Carmelites respectively. There are lots of others who don’t advertise their status in this way, or who are Benedictine Oblates and so on.
My parents are secular Franciscans - they’d see their role partly as supporting their Franciscan priests, monks and nuns in their work where possible. Spiritually with their prayers if they can’t give any material help.
Lay members of an order have certain extra daily prayers and other obligations above what the average layperson does (the SFO go on spiritual retreats a few times a year, attend monthly meetings and so on). The extent of these depends upon which organisation you join.
Basically they follow a modified form of the religious rule of life that priests, monks and nuns follow. For the Franciscans, for example, a commitment to poverty (detachment from possessions) is an important part of the way of life. So SFOs make a promise of poverty, but this will be much less extreme than what monks and nuns are bound to.
The purpose? Well, it’s a way of being part of the order whose spirituality and charism you identify with, whilst still having some liberty - to pursue career and have a family for example.
It’s highly unlikely my folks could be doctors (or at least not full-time) if they were respectively a priest or monk and nun. And they certainly couldn’t have had and raised their four beautiful daughters … and me (see, that Franciscan spirit of humility is catching)
