I'm not sure if this is the right place to be posting this but here goes anyhow...
The three benefits that stand out to me are:
Regular agreed times- with times prearranged time is set aside and prioritised for the sessions well in advance
Responsive and flexible to needs- the mentee drives the sessions "what's on top for you?"
Self tailored by Mentee- Mentee goals are self directed and there is no right or wrong goals to have.
A limitation may be that as a mentor the boundaries need to be maintained in terms of time and the temptation to always be responsive outside of "session" times.