Why transparency so often turns black
The standard way to convert an image in a browser is to draw it onto a canvas and ask for JPEG bytes. A fresh canvas is transparent, JPEG cannot express transparent, and the encoder resolves those pixels to black. Nothing has gone wrong in any individual step - the result is simply a logo with a black box where the background should be, and it happens to anyone who does the obvious thing.
This converter fills the canvas with white before drawing the image onto it. White is the right default because the overwhelming majority of transparent PNGs are logos, icons and diagrams intended to sit on a light background, and white is what they were designed against. It is a one-line difference that decides whether the output is usable.
The conversion is worth doing for photographs and little else
A photograph saved as PNG is usually several times larger than it needs to be, because PNG is lossless and stores every gradient in full. Screenshots of photographic content, exports from tools that default to PNG, and images pulled out of documents are all routinely enormous for no benefit. Converting those to JPEG at a sensible quality is a large saving nobody can see.
The reverse is true for anything with text, flat colour or sharp edges. JPEG compresses by discarding high-frequency detail, and a sharp boundary is nothing but high-frequency detail - which is why text in a JPEG grows a faint halo and a flat background develops blotches. Keep diagrams, screenshots of interfaces and logos as PNG.
Quality settings, and where the useful range is
The quality number is not a percentage of anything meaningful, and the curve is steep. Between the top of the range and the mid-eighties, the file gets substantially smaller and almost nothing visible changes. Below about seventy, artefacts start appearing around edges and in flat areas, and they never go away because the damage is written into the file.
Somewhere in the low-to-mid eighties is the usual sweet spot for photographs on the web. The honest way to choose is to watch the output size as you move the slider and stop at the point where the next step down stops buying much - which is a thing you can only do comfortably when conversion is instant and nothing is being re-uploaded each time.