So the herders can hear them if they stray off.

Because they can't use tamborines.

Because if they didn't, they'd be laide cows. (Sorry...French joke. Just had to get it out there.) :P

So if they get lost they can be heard and found.

because it would be very hard for them to carry one ;)

So the farmer can hear if they are being tipped

It makes the girl cows look sexy and makes the boy cows want to reproduce. lol ;)

Because as they graze the hills the cow-herder can hear where they are and if they are still close by and then when it's time to take them home he can find them by their sound.

So an inexperienced hunter doesn't shoot them. This really happens!

They are reindeer trainees for Santa Clause.

..so they can't sneak up on their adversaries.

So they can't sneak up on squirrels.

To show that they don't fear the reaper.

They tried buzzers at first but they ended up going through batteries like crazy.

1) "While the cowbell is commonly found in musical contexts, its origin can be traced to freely roaming animals. In order to help identify the herd to which these animals belonged herdsmen placed these bells around the animal's neck. As the animals moved about the bell would ring, thus making it easier to know of the animal's whereabouts. While bells were used on various types of animals, they are typically referred to as "cowbells" due to their extensive use with cattle. - Almglocken / Alpine Bells Almglocken, sometimes known as Alpine Bells, typically refer to bulbuous brass bells that are used to play music as a novelty act or tourist attraction in the northern Alps. Since they are tuned differently to distinguish individual animals, they can be collected "from the pasture" in random tunings, but commercial sets in equal temperament are also available. The metal clapper is retained, and they sound much more noisy than handbells, which are otherwise used similarly in ensembles. Composers who included almglocken among their musical palette include Tōru Takemitsu, Gustav Mahler, Roy Harter, and Karlheinz Stockhausen." Source and further information: http://en.wikipedia.org/wiki/Cowbell 2) "In the Early Middle Ages, bells were mostly reserved for religious purposes. In the High Middle Ages to the 14th century, they became popular also in secular pageantry such as knightly tournaments. The earliest attestation of bells used for livestock dates to the late 14th to early 15th century (Grimm's Deutsches Wörterbuch s.v. "Kuhschelle" points to a 1410 mention in a Frankfurt archive; the OED lists 1440 as the earliest attestation of bell-wether, the leading sheep of a flock, on whose neck a bell is hung, and attributes the phrase "to bear the bell" in the sense "to take the first place", originally referring to the leading cow or sheep of a drove or flock to Chaucer's Troilus and Criseyde, 1374). In the 15th century, a cow bell was this a luxurious item, worn only by the best and leading piece of livestock, and the wider distribution of the bell worn by livestock is a gradual process of the Early Modern period. Rabelais in the mid 16th century in his Gargantua and Pantagruel makes this explicit, stating that "such was the custom, to appear on the field wearing jingling garment, as the high priest wears when entering the sacristy; since the tournaments, that is, the contest of nobility, have been abolished, carters have taken the bells and hung them on their hacks" (cited after Grimm, s.v. "Schelle"). Swiss folklore reflects a period when a great Trychel was a rare and much-coveted item. Thus, a legend of the Simmental tells how a young cowherd strays inside a mountain, and by a beautiful woman is offered the choice between a treasure of gold coins, a golden Trychel, and the fairy herself chooses the Trychel." Source and further information: http://en.wikipedia.org/wiki/Trychel

Because their horns do not work!

so their owners know where they are in the fields

Because they slip off their udders?

so when they walk you can hear them so they dont run awy or wander off without Farmer Brown here knowing it

Same reason you put a bell on a cat... to scare off the birds.

Because their horns don't work?

Because their Horns don't work! Wakka Wakka Wakka!

so they know if they are being ubductated