Who needs one:
Companies raising under Tier 2 (up to $75M) — two years of audited financial statements are required by SEC rules before your offering circular can be qualified. No audit, no raise.
Companies raising under Tier 1 (up to $20M) — audited financials are not federally required, but most state regulators and serious investors will expect them. Skipping the audit creates friction with both.
Companies that have already filed or are amending a prior offering — if your financials have changed materially, updated audited statements may be required before requalification.
Sources: 17 CFR §230.251 (Regulation A offering tiers and limits); 17 CFR §230.257 (Tier 2 audited financial statement requirement); SEC Release No. 33-9741, Regulation A+ Final Rules (March 2015); PCAOB AS 1000 / U.S. GAAS (applicable auditing standards for Reg A+ issuers).