“[Mt 23:2] is normally taken to mean to have authority to interpret for people the demands of the Mosaic Law. But this is difficult, given the fact that the same people to whom the sitting is applied are identified soon after in v16 (cf. 15:14) as ‘blind guides.’ Powell identifies ten different approaches to dealing with this tension, but as he clearly shows, none is satisfactory…So, what is the force intended by ‘sit/sat in the seat of Moses’? According to Powell,
Jesus may be simply acknowledging the powerful social and religious position that [the scribes and Pharisees] occupy in a world where most people are illiterate and copies of the Torah are not plentiful. Since Jesus’ disciples do not themselves have copies of the Torah, they will be dependent on the scribes and the Pharisees to know what Moses said…In light of such dependence, Jesus advises his disciples to heed the words that the scribes and Pharisees speak when they sit in the seat of Moses, that is, when they pass on the words of the Torah itself.
We might say that the scribes and Pharisees were walking copies of the Law. What they did with it might be suspect, but not their knowledge of it. They could be relied on to report the Law of Moses with care and accuracy,” J. Nolland, The Gospel of Matthew (Eerdmans 2005), 922-23.