The main benefit of a partial accreditation certificate is that you may well be exempt from some of the PPL exams. Also, your previous training will effectively be certified to a known standard and the CFI deciding your training will be able to rest assured that you have been trained accordingly. Thus your conversion may be slightly quicker.
Note that the PPL requires rather more medium level visual navigation than you have probably been used to - and you will certainly need to be able to use a Dalton computer for the navigation exam and for pre-flight navigation planning as MDR will not enable you to make sufficiently accurate ETA estimates for the PPL Navigation exam or PPL Skill Test navigation section.
Quite why 1FTS don't give you this information whilst you're training with them is something which you need to take up with them.