I think it achieves two things:
It removes the friction of travel-time as a bottleneck meaning that active tutors can teach more lessons per day increasing Tutorful’s commission.
It lowers the price for clients making them more amenable to buying (more) lessons. The change forces us to compete with the entire population of tutors which means lowering rates to stay competitive. This particularly sucks for tutors based in cities like London where most agencies won’t pay you less than £35 p/hour. But these are often considered ‘London rates’, and it isn’t unusual to have graduates charging £15 p/hour outside of major cities.
This is what unions are for!
Edit: Perhaps they also have data which suggests that people are less likely to bypass Tutorful when the arrangement is online only. If all communication were on-platform which seems to be the goal, it would be harder for tutors and clients to make alternative arrangements such that Tutorful doesn’t get a cut. Certainly removes the potential for clients to offer to pay in cash upon a home visit.