We already know that the pronoun le can replace a masculine noun (Replacing nouns with le, la, l', les = it, him, her, them (direct object pronouns)) or a whole idea (Using le or l' to refer to previously mentioned ideas (direct object pronoun)).
Now look at these sentences:
Note that in sentences when you don't want to repeat an adjective, in French you'll need to use the object pronoun le to replace it.
You can't just omit it, and say -> Ce canapé est confortable mais ces fauteuils ne sont pas (quoi?). In French, something is missing here.
Note also that le becomes l' in front of a vowel or a mute h.
ATTENTION: In this context, the pronoun le never agrees in gender or number.
Learn more about these related French grammar topics
Examples and resources
Q&A Forum 1 question, 2 answers
Ils sont fatigués et je l'est aussi is wrong because the verb "est" does not match the subject "je". The second sentence is correct:
Ils sont fatigués et je le suis aussi. -- They are tired and I'm (tired) too.
-- Chris (not a native speaker).