Because AAC is a lossy compression format, it achieves file-size reduction by discarding an enormous amount of data. Once gone, that data cannot be recovered.
When you convert that AAC file (or any lossy file, for that matter) to a WAV file, you will get a much larger file that sounds as good as the AAC original, no better. The WAV file is simply a representation of what was in the AAC file, and that means the data lost in compression to AAC is not restored. On the other hand, conversion to WAV discards no data, so this conversion should not degrade the sound quality of the file beyond the damage done by the original conversion to AAC.
If you then choose to re-convert that WAV file to a lossy compressed file, however, you will subject it to lossy compression all over again, and you will lose additional data.
There are perhaps good reasons to convert a lossy-format file to WAV format under certain circumstances. The ability to play the file on devices that don't support AAC is one such good reason.
This is in contrast to conversion of a lossless compressed file -- such as a FLAC file. By design, FLAC compression does not discard data, so when you convert a FLAC to a WAV file you get back the original file from which the FLAC was generated. This applies to all lossless formats, by definition.
