Homes have a few elements right outside that need to be covered up with an outdoor privacy screen. No one can build a very small minifence around the electric box or the compost pile. No one will be able to get close enough if there is a permanent fence there. Creating protection from prying eyes in bedroom windows or garbage cans is common. If you take pride in your yard to can cover any thing seemingly unsightly like your garbage cans or the air conditioning compressor. This way the only things in your yard are natural and we can cover up the mechanical devices or the trash. The best part is when you are needing to get to what's covered the screen is lightweight and can be removed temporarily.
Building a real fence around these common things has a bunch of cost and hassle associated with it. You'd have to make the enclosure larger than the object so that access was not prevented. You'd have to spend a lot more than a portable fence because you have to buy all the wood and nails and concrete and paint or stain that it would take to finish the job. Digging holes for fenceposts are not very fun.
Privacy screens are easy to setup and move around. They can clean up the last touches on your garden to make it yard of the month.