He kept some for the farm.And it seems he used the same ones for riding- or at least Jane and possibly others, excepting lLzzie) did so too.

So why did JA make this remark?Was it to show that the Bennet's were not possessed of a sufficient income to warrant owning horses merely for riding?Perhaps.

But there is also another point to consider.

Horses that were for personal use only were subject to a tax.(as were most things in war torn late 18th century England) There was the horse tax to consider. It was high, and was continually being raised. In 1797 the tax was 54 shilings(per horse I presume).

However, it was not levied on horses used in husbandry(farming) and so perhaps, whereas in Emma, Mr Knightley had horses for this purpose but was too honourable to then use them for carriage horses ;

She( Emma)followed another carriage to Mr. Cole's door; and was pleased to see that it was Mr. Knightley's; for Mr. Knightley keeping no horses, having little spare money and a great deal of health, activity, and independence, was too apt, in Emma's opinion, to get about as he could, and not use his carriage so often as became the owner of Donwell Abbey. She had an opportunity now of speaking her approbation while warm from her heart, for he stopped to hand her out.
"This is coming as you should do," said she, "like a gentleman. I am quite glad to see you."

but Mr Bennet,was not so scrupulous in his affairs and used his work horses for personal use and thereby avoided the tax......possible,all too possible.

Muriel Smith in the Jane Austen Society's Report of 1975 speculates that this is why a spendthirft like Mr Bennet had horses for farm use, and also used them for carriages.He was having them assesed as horses used soley for farming for tax purposes, whilst using them for recreational purposes on the sly!!