PNG, WAV and FLAC are lossless in the paths this site exposes. JPG, WebP lossy, MP3, AAC and Vorbis are lossy. A bitrate control belongs on the lossy path. WAV and FLAC do not get a fake bitrate slider.
Converting WAV to FLAC can shrink the file without changing the samples, if the encoder is present. Converting MP3 to WAV does not improve the audio. It only decodes the MP3 you already have.
Use lossless while you edit. Use lossy when you need a smaller file for delivery and you have checked the result.