1. To stretch out content. If not for a lock out, people would get full sets of gear + alt gear in less than a month. Due to getting so much gear, they'd also rapidly progress through second coil, making it nearly pointless within a few months time. They'd quickly grow more tired of the game. It's mainly a artificial barrier of progress.

2. People would sell runs.


If you're talking about making it like how CT gear used to be, one gear per week lockout instead, that would also do the same as number 1. on the list. You'd fully gear out a character within 8-9 weeks, faster if you count oils / sands. People would also sell runs in this instance.