In the modern era in developed countries squatting is now largely just trespassing, which is illegal, but there have been periods where that wasn't the case.
Most notably, following the two World Wars you had a large number of young men (primarily) that died overseas with corresponding effects like their family might move as a result and so on. So you had a large number of vacant properties with no clear idea if the owner was still alive or not. So squatting became a way of "solving" that problem. A squatter could get the rights to a place if they occupied it for some long period of time (typically over 10 years) if no one showed up earlier to claim ownership.
In the computerized records era, and with no mass casualties from war in developed countries, this is now relegated to an historical anachronism.
Most notably, following the two World Wars you had a large number of young men (primarily) that died overseas with corresponding effects like their family might move as a result and so on. So you had a large number of vacant properties with no clear idea if the owner was still alive or not. So squatting became a way of "solving" that problem. A squatter could get the rights to a place if they occupied it for some long period of time (typically over 10 years) if no one showed up earlier to claim ownership.
In the computerized records era, and with no mass casualties from war in developed countries, this is now relegated to an historical anachronism.