Why convert PNG to WebP
Almost always page weight. PNG is lossless, which is exactly what you want while you are working and exactly what you do not want on a live page. A hero image saved as PNG can easily be 3 MB where the same image as WebP is 300 KB, looking identical at any size a visitor will see it.
WebP is the format that lets you make that trade without giving up transparency. That was the historical problem: JPEG was small but flat, PNG kept the alpha channel but was heavy, and there was no option that did both. WebP does both.
Browser support is no longer an argument
This used to be the objection, and it is out of date. Chrome shipped WebP in 2010, Firefox in 2019, and Safari in version 14 in 2020, which also covers iOS 14 and later. Edge inherited it from Chromium.
In practice that means every browser in current use reads WebP. The remaining gaps are in desktop software rather than browsers, which is the reverse of the AVIF situation and why WebP is the safer choice for web output today.
Quality, and when to leave it alone
The slider controls a lossy encoder, so 100 percent is not the same as the original file. At 92 percent, which is the default here, most images are impossible to tell apart from the PNG in a side-by-side comparison, and the size saving is already most of what is available.
There is one case that deserves care: flat graphics with hard edges, screenshots of text, and pixel art. Lossy compression of any kind puts faint ringing around high-contrast boundaries, and on a screenshot that is visible. For those, either push quality to 100 or keep the PNG.
Keep your source file
Because the output is lossy, the WebP is a publishing artifact, not a working file. Convert from PNG each time you need a new size rather than editing the WebP, or you will stack generations of compression on top of each other.
Local, private, no queue
Conversion happens through the canvas encoder built into your browser. Nothing is sent anywhere, which also means the only limit on a batch is your own machine.