Bold and italic are combined into a single dedicated Unicode block, so this tool produces both effects at once from one conversion rather than requiring you to apply bold and italic separately.
Unicode's Mathematical Alphanumeric block includes a distinct set of characters that are bold and slanted simultaneously, separate from the plain bold and plain italic sets — which is what makes a clean one-step conversion possible here.
Both the full uppercase and lowercase alphabet are covered without the exceptions seen in some other styles (like italic's lowercase h), since this block was defined completely.
Bold italic reads as strong emphasis — useful for a headline-style line in a bio or a stand-out word in a caption.
It reads slightly more emphatic since it combines two visual cues, but which looks better depends on your font rendering — try both bold and bold italic side by side.
No, Unicode doesn't define bold italic digits, so numbers in your input stay unchanged.
Generally yes, since it's standard Unicode, though very old devices with limited font coverage may show blank boxes for a few characters.