To protect yourself from and fight against weakened fractions of the government.

The military cannot occupy the entire space of the US so maintaining order of a tyrannical government would require a low ratio of military guards to citizens. With even 50% of the citizens potentially armed and willing to fight back, it's an untenable position for the government.

A militia could fight just as effectively as the Taliban against the government and be several orders of magnitude larger. The US military is good at eliminating identified enemies, but it's pretty terrible at nation building and that's even in its strongest state.