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I have seen firms (good ones) advertising vacancies but never getting to fill their vacancies.
If a firm needs new attorney(s), must it hire? or do firms hire only when they find the right candidates?
alternatively, if the firm is not actively looking or in the hiring mode, will it take on new attorney(s) when someone suitable comes along?

Need or projected need. Hiring is a business decision. If trying to get into a new practice area, then lateral hiring or mergers.