9: Among the greatest sages of Israel were wood-hewers and water-bearers; some were blind; nevertheless they involved themselves in TT during the day and at night; and this group includes the bearers of the tradition all the way back to Moshe Rabbenu (Moses our Teacher).

Q1: This entire Halakha seems superfluous; we alrady know that every man is obligated.

YE: Perhaps this is a continuation of the previous Halakha and is both exhortation and modeling for anyone who wants to claim that they are too busy. Also, see the Gemara in Yoma (35b) of how Hillel destroyed any poor person's defense for not learning.

Q2: Why add that some of these manual laborers and blind men were bearers of the tradition?

YE: This is the exhortative part - that not only are we all obligated in study, but noone can claim that there is no value to learning since he won't achieve greatness due to his poverty/work load etc.

Rambam, Copyright (c) 1999 Project Genesis, Inc.