Open wiring applies mostly to outdoor pole mounted distribution systems. Junction boxes are seldom used.
For interior wiring almost all splices, taps, joints and everywhere that the insulation on the conductors is removed must, by code, (in North America) be enclosed by an approved junction box.
Junction boxes must be accessible for service and trouble shooting.
The advantages are safety from shock and fire, protection of the connections, and, if the code calls for a junction box, the AHJ will require that the installation be modified by the installation of a junction box(s).
Bill
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"Why not the best?"
Jimmy Carter