God commanded that every man in Israel must appear before him three times each year (23:17). They must appear at the tabernacle (also called the Tent of Meeting). While Israel camped around the Tent of Meeting in the wilderness, this command was not difficult to obey. The people lived close to the tabernacle.
Later, after Israel settled in the promised land, many people lived far from the Tent of Meeting and later from the temple in Jerusalem. Travel became longer and more demanding.
This command was likely meant to prevent the people from building local worship centers. If each region created its own place of worship, the nation could become divided.
Local shrines could also allow pagan influences (beliefs and practices from other religions) to enter the worship of Yahweh. By gathering at one central place, the people would remain united in their worship of the one true God.
Sadly, the people did not always carry out these commands carefully (see, for example, 2 Kings 23:21–23).