Where TGA came from
Truevision created TGA in 1984 for its early graphics cards, and its simple structure plus reliable alpha channel made it a fixture in game development and 3D rendering. Unreal, Unity and most DCC tools still import it happily, which is the main reason a format from 1984 remains in active use.
The technical picture
Compression: Uncompressed or lossless RLE. Transparency: Full 8-bit alpha. Animation: Not supported. Colour: 8-bit per channel. Typical file size relative to WebP: many times larger than WebP.
Where it opens
Game engines and 3D tools; poor elsewhere. This is usually the deciding factor when choosing between TGA and WebP — WebP renders in every browser but is uneven in desktop software, while TGA has a very different support profile.
Converting TGA to WebP
TGA's minimal compression makes it far too heavy for the web. WebP keeps the alpha channel and cuts the size dramatically. Expect a large output file — TGA barely compresses.
Converting WebP to TGA
Game engines, texture pipelines and 3D tools often want TGA for its predictable uncompressed layout and clean alpha channel. Expect the TGA output to be many times larger than WebP — that difference is the price of getting a format the destination will actually accept.