Burst stills usually give higher-quality frames than video grabs.

Key advantages:

• higher resolution: video frames are often much smaller than full-resolution stills. For example, 4K video is only about 8MP, while many still cameras capture much more, which helps with detail and cropping.
• less compression: video is typically compressed heavily, often using frame-to-frame compression. A single extracted frame usually has less image quality than a still photo.
• raw capture: photographers often want RAW files for maximum editing latitude. Most cameras don’t record RAW video in normal use, and doing so creates huge files.
• data/storage limits: full-resolution stills at video frame rates would generate enormous data rates, making storage and processing impractical in affordable portable cameras.
• shutter/camera design: still cameras are built to shoot short high-speed bursts, not to deliver unlimited full-quality still frames continuously.

There are also practical factors: still-photo gear is generally cheaper and simpler than high-end video systems, and flash photography is easier with stills than with continuous lighting.

Video frame grabs can be useful, especially when resolution needs are modest, but for the best single images, burst still photography is usually the better tool.