Walled towns/cities used to be common. In some contexts, it makes more sense than walled countries.

People build structures to form their communities. Not everyone from outside the community can be trusted to respect the integrity those structures, especially when a rival community builds an army or if there are roaming bands of raiders or whatever. In those situations, a walled town becomes necessary for the common defense, provides a refuge for the surrounding villagers, and overall just makes it a lot easier for people to protect themselves.

Not only that, it's just much more practical materially. It's easier to build and man a wall one mile in circumference than it is to build one 500 miles long with no closure.

In the modern context, walled towns aren't really as necessary as they were in say medieval times when basically any land outside a fiefdom was more or less unpatrolled and most places didn't have a unified body-politic maintaining civic order.

However, as society breaks down, communities polarize, extremists turn to political violence, and law enforcement agencies no longer feel obligated to protect people, a time may come again when building a wall around your town or neighborhood and controlling access points may become useful. Especially in say a post-apocalyptic scenario where there's a complete breakdown of society and you can no longer trust that the people in the next town over or the trailer park beyond that aren't gonna bring violence to your door.

Of course now there's aerial technology which can defeat the purpose of a wall, but it might at least keep Johnny Redneck with his extra big-ass truck and AR-15 out of your town. And nets and things might snare drones before they can detonate...