why does gulliver think the account ofstruldbrugs to be uniqe and a potent source of entertainment to the readers ?

I don't see any evidence of Gulliver believing the account to be entertaining for readers. Gulliver is appalled by his own initial visions and the stark reality of their lives.

The reader will easily believe, that from what I had hear and seen, my keen appetite for perpetuity of life was much abated. I grew heartily ashamed of the pleasing visions I had formed; and thought no tyrant could invent a death into which I would not run with pleasure, from such a life.

This question pertains to Chapter Eleven. The answer from the text follows:

I thought this account of the struldbrugs might be some entertainment to the reader, because it seems to be a little out of the common way; at least I do not remember to have met the like in any book of travels that has come to my hands: and if I am deceived, my excuse must be, that it is necessary for travellers who describe the same country, very often to agree in dwelling on the same particulars, without deserving the censure of having borrowed or transcribed from those who wrote before them.