for Harriet.

We saw how Emma thinks it best to have Harriet with her after her refusal of the proposal.
'To be kind' Emma tells herself, but 'perhaps' (as others have said) also to keep a check on Harriet from changing her mind by sharing this with someone, and getting a different kind of advice.
(BTW I'm curious if Harriet could follow up on the proposal after the refusal).

Now in CH:13 Harriet is indisposed.
The ON tells us that;

"The evening before this great event (for it was a very great event that Mr. Woodhouse should dine out, on the 24th of December) had been spent by Harriet at Hartfield, and she had gone home so much indisposed with a cold, that, but for [her] own earnest wish of being nursed by Mrs. Goddard, Emma could not have allowed her to leave the house."