why do we build fences in are back yard?

is it for the neighbors not to see what we are doing

That and to keep your neighbors cat from defecating in your yard.

There are several reasons. One, do you not leave things out and around your back yard? Things like barbecue grills, hoses, toys, etc.? If you don't have your yard fenced, it's just there for the taking. When your children are playing, do you want a stray dog, wandering into the yard and attacking your kids? Do you want strangers to have easy access to your family members and/or house? And yes, there is the privacy issues. For instance, you have a pool and don't want people gawking at you. Or someones child wandering in and drowning. The reasons can go on, but I think you get the point.

yeah. plus to keep unwanted people out

Privacy, security, the freedom to walk around in your own damned yard and pee off the front porch, to keep the dogs from eating neighborhood children... at least that's why I walled my acreage in. And to those of you who feel that building walls is an unfriendly act, I say to you, "Think what you like, I don't wish to have people like you for friends anyway".

privacy.

I have a pool/spa and I don't want neighbors watching. Also have dogs and I don't want them to get out.

And to keep your neighbor from stealing your food and crapping in your yard. Plus, what if he pitches a tent and moves in?