I am on placement as a teacher and today a student asked me this but I didn't have a good answer. I told them I would ask historians and get back to them.
"It seems like the most useless gift to give somebody. Surely the Trojans were confused by it but had to pretend they liked it. Like when grandma gives you an ugly shirt but you have to pretend you like it, until they're gone."
It wasn't a gift to them, they stole it (well were kinda tricked into stealing it).
It was built as an offering to the god Athena. The Greeks then abandoned the horse and made like they were giving up and leaving. The Trojans pulled the horse in to take the Greek's offering as their own to gain godly favor.
To be fair, a true answer to this is almost impossible to give. History from this time kinda comes in the form of half myth half truth and was written about many centuries later. The main written source of this war comes in the Iliad which was written around 760-710 B.C. This was written roughly 400 years after the war is thought to have occured. Because of this difference almost all sources must be taken with a grain of salt. As well, it appears that Troy was almost completely sacked after it was captured so the true reason why the accepted the horse may never be known as it is likely that those who made the decision were killed in the aftermath of the fall of Troy. In short, no one can really say why they took the horse or if there ever really was a horse or if it is just a legend.
In the story itself, the horse isn't a gift to the Trojans, it's supposed to be an offering to Athena to ensure safe return. The Greeks build it, then pretend to sail away; it's then captured by the Trojans as a victory trophy (note that the horse is the emblem of Troy). You know what happens next.
One possible historical explanation was that the horse was actually a siege engine. Some battering rams at the time were indeed built to look like animals, and covered in hide to protect against fire.
But really, we don't know. There isn't a historical consensus over the historicity of the Iliad. Excavations in the Hisarlik/Troy site have indeed revealed that the city was destroyed by fire somewhere around the 12th century BC, and some archaeological finds seem to support an armed conflict, but it's pretty much impossible to know how much of the Iliad is historical truth, how much is oral tradition and how much is literary creation.