Alpha in lossy mode
This is the unusual part. JPG has no alpha at all, so a transparent logo must be stored as PNG — which means lossless compression and a large file. WebP lets you have soft, anti-aliased transparency and aggressive lossy compression at the same time.
Alpha in lossless mode
Lossless WebP stores the same 8-bit alpha as PNG with identical fidelity, in a file about 26% smaller. For icons and UI assets this is a straight upgrade with no trade-off.
How the alternatives compare
PNG: full 8-bit alpha, universal support, large files on photos. GIF: 1-bit only — every pixel is fully opaque or fully invisible, which produces hard jagged edges. JPG: none whatsoever. AVIF and HEIC both carry full alpha.
What happens when you convert away
Converting WebP to PNG, AVIF, TIFF or HEIC preserves transparency. Converting to JPG or BMP composites it onto a background colour — white unless you choose otherwise, which is rarely what a logo wants. Converting to GIF reduces it to 1-bit.