Automated scanners are fast, consistent, and well suited to catching objective, rule-based issues at scale, which makes them valuable for continuous monitoring and for catching regressions as a site changes over time. Their major limitation is coverage: independent research consistently finds automated tools alone catch only a minority of all real WCAG failures, since many criteria (like whether alt text is meaningful, or whether reading order makes sense) require human judgment to evaluate.
Automated testing works best as a fast, repeatable first layer, integrated into development workflows, backed by periodic manual and real assistive-technology testing rather than relied on alone.