When you actually need WAV
Some tools still want uncompressed audio: older video and audio editors, CD burning programs, hardware samplers and DJ gear, broadcast playout systems and some game engines. Converting once to WAV avoids decoding on every edit and makes sample-accurate cuts easier.
If the software you use plays MP3 directly, you do not need to convert — the MP3 already is the best copy you have.
WAV will not improve the sound — and it is much bigger
MP3 compression removed detail when the file was made; WAV stores the decoded result exactly, no more and no less. The file grows to 10.58 MB per minute at 16-bit/44.1 kHz stereo: about 11 times a 128 kbps MP3 and about 4.4 times a 320 kbps one.
That size increase buys compatibility, not quality. Keep the MP3 if it is all you need for listening.
Exact length: encoder delay removed
Every MP3 starts with roughly 1,100 samples of encoder delay (about 25 ms at 44.1 kHz). When the MP3 carries a LAME or Xing header, as files from most modern encoders do, the browser removes that delay and the end padding, so the WAV has exactly the length of the original recording.
Very old MP3s without such a header keep the short silence at the start; the result card shows the sample count so you can check the length before aligning audio in an editor.
Sample rate and bit depth
The WAV uses the MP3's own sample rate (usually 44.1 or 48 kHz) and 16-bit samples by default, which is the CD format and what almost every tool accepts. Choose 48 kHz if a video project uses that rate, so the editor does not resample on import.
24-bit keeps the decoder's extra precision but makes the file 1.5 times larger with no audible benefit for MP3 sources. Loud MP3s can decode slightly above full scale; any samples that had to be limited are counted in the result.
Tags from MP3 to WAV
ID3 titles, artists, albums, track numbers, years, genres and comments are copied into the WAV's RIFF INFO list. Cover art and album artist have no place in WAV and are left out, as the result card notes.
If you later need a tagged lossless file, convert to FLAC instead; FLAC keeps tags and cover art in a widely supported format.