WAV for production and editing
WAV is a container that is commonly used for uncompressed PCM audio. That makes it a predictable working format for recording and editing, at the cost of much larger files than compressed delivery formats.
MP3 for convenient delivery
MP3 removes audio information to reduce file size. At sensible settings it is convenient for listening, sharing and devices where storage or bandwidth matters more than retaining a production master.
Converting MP3 to WAV does not restore lost detail
Turning an MP3 into WAV can make it compatible with a WAV-based workflow, but it cannot recreate information discarded by the earlier lossy MP3 encoding. Keep an original high-quality source when quality matters.