Not that you ever need to do this, of course - there's no reason why you can't just type in A and Z in your HTML, without having to jump through the hoops of encoding. How unfortunate, therefore, that whoever designed this ad didn't have a similar basic knowledge of HTML markup. HTML entities are a way of encoding special characters, like accents and symbols, in a way that is readable by all machines and will not get corrupted during transmission.
Numeric markup cuts out that lookup stage and specifies the raw character code directly.
Bypassing any sanity-checking by the browser is risky, but can be necessary if the browser doesn't have a name for the character you want to display. Such a basic misunderstanding of entity coding is unforgiveable for a site that pretends to be an authoritative source on HTML markup.