I think its just to be pretty.

Tack gives a small bonus for halter and discipline shows. 

@KayTea it's nice you're trying to help, but please refrain from giving incorrect information. If you're not sure of an answer, check the Wiki or leave it to another player :)

And I think encrusting might give more of a boost. Limited edition tack has great bonuses too!

The wiki has all kinds of answers to your questions, although some still needs updating. The tack page links a spreadsheet that has most of the tack/dyes/minerals etc and you can see costs and worths plus bonuses of various items as well.