The phrasal verb "sit down" means "be seated/take a seat/sit".

So why do we need to say "down"?

The phrasal verb "sit up" does not mean sit down or sit sideways.

Well, despite the preceding nonsense (which I take full credit for), these two phrasal verbs are interesting, read on:

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1  Take a seat, as in, "Please, sit down? I won't be long."

2  "Sit down to..." which means to prepare to eat a meal/sit at the table, as in, "At six we all sat down to dinner."

1  Rise to a sitting position from lying down, as in, "The sick child sat up and asked for a drink of water."

2  Stay up later than usual, as in, "The nurse sat up with the patient all night."

3  Sit with the spine erect (good posture), as in, The teacher always told the students to sit up straight.

4  Become suddenly alert, as in, "The students sat up when they heard the alarm.

The same sense appears in the related expression 'sit up and take notice', as in,

"When he mentioned the arrival of a movie star, they all sat up and took notice.

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