When to convert AVIF to PNG
Two situations, and they are different enough to be worth separating.
The first is compatibility. AVIF is new, and while every current browser reads it, most desktop software does not. If a design tool, an asset pipeline or an upload form is rejecting your file, PNG is the safest format in existence: there is essentially no image-capable software that cannot open one.
The second is editing. AVIF is normally lossy, which makes it a poor working format. Every time you open it, change something and save, the codec runs again and takes another small bite out of the picture. PNG is lossless, so once you convert, the file stops degrading and stays exactly as it is through any number of edits.
Why PNG rather than JPG
Because AVIF can carry transparency and JPG cannot.
If your image has a cut-out subject, a soft drop shadow or anti-aliased edges against nothing, converting to JPG forces all of that onto an opaque background colour. PNG keeps the alpha channel intact, so the transparency comes through exactly as the AVIF stored it.
If the image is a plain rectangular photograph with no transparency at all, JPG is the more sensible target and will produce a far smaller file.
Be ready for the size
This is the largest size increase of any conversion here, commonly five to fifteen times. A 300 KB AVIF landing as a 4 MB PNG is entirely normal.
The reason is that the two formats have opposite goals. AVIF discards whatever it judges to be imperceptible; PNG describes every pixel exactly and never discards anything. You are paying bytes for exactness and compatibility, which is a fine trade for an editing source and a poor one for a web asset.
Everything runs locally
The AVIF decoder is already inside your browser. This page uses it, draws the result onto a canvas, and writes the PNG in the same tab, so your files stay on your machine throughout.