Convert only if required by very old software from the 1990s-early 2000s that can't handle modern formats. For most purposes, converting WebP to PNG is far better - PNG is lossless like BMP but 50-75% smaller with universal support.
BMP files will be 10-20 times larger than WebP. A 500KB WebP might become an 8-10MB BMP. WebP has excellent compression while BMP has none, creating enormous file size differences for no quality benefit.
No. If the WebP was lossy, converting to BMP won't recover lost quality. BMP will faithfully store the quality from WebP without further degradation, but can't improve it. For lossless WebP, the BMP will be identical quality but much larger.
BMP is needed only for extremely rare cases: software from before 2000 that lacks PNG support, certain industrial/medical legacy systems, or specific professional workflows. For 99% of users, PNG provides the same benefits as BMP with much smaller files.
Use PNG instead. PNG is lossless like BMP but with 50-75% smaller files and universal support across all modern software. PNG does everything BMP does but better. Use BMP only if you absolutely must for specific legacy software requirements.