A piece of land comes under the control of the government through certain ways. It may take place through the following methods:
In USSR there was a time when the political ideology of the country was socialism and therefore it practiced state ownership of land. The major portion of land was owned by the government. In other socialist countries the state owned half or more the half of the land.
There are quite a few advantages of state ownership of land. It is a good practice since the land gets distributed equally among the people. If the state owns a land then it is possible to reduce exploitation of the poor people by the rich . In private ownership an owner could impose any amount of rent on the tenants but this is not possible under the state ownership because the state fixes a rent that is paid by the farmers plowing the field. Sometimes it so happens that there is a barren piece of land which is of no use to the common people. In such cases the government interferes and owns the land.
The Government also leases out land for the purpose of farming.
In many of the Catholic countries the Church owns a huge amount of land which it uses for ecclesiastical purposes.
In US state owns forest land or a park. It might also be a grassland or a swamp which is not of common use. The land on which there is an office building or prisons is not owned by the government. The land on which a military base has been established is also a government undertaking.