Well Phrase vs. Font elements

Phrase Elements
abbr, acronym, em, strong, dfn, code, samp, kbd, var, dfn. ins and cite
 
Font Style Elements
tt, i, b, big, s, u and small

Phrase elements allow to apply meaning to the text, more of a structure. The bottom line is that when we look at a button with a 'B' I expect <b>.