Both formats store pixels, but their image details can differ
TGA is used in some graphics and rendering workflows and may carry alpha or a particular pixel depth. PNG is widely supported for web and general image use. The conversion changes the container and may normalize how pixel information is represented.
PNG uses lossless compression for supported image data, but a conversion cannot restore color information already absent from the source. Keep the TGA master when it is part of a production pipeline or contains data needed by another tool.