Small Caps fonts are to be distinguished from All Caps. All Caps are used at the beginning of a sentence, at proper nouns, at the beginning of all nouns in some languages. Small Caps are used for headlines, abbreviations, entering the first word or a sentence in a long paragraph.
Of course, it is quite possible to take semibold font and reduce its size till 70 percent for "plausibility". But this is "mauvais ton" for professional typographers and designers. They use only Small Caps fonts designed especially for these purposes.
But what is the difference? Small Caps and scaled capital letters will look the same for someone who is not a specialist. However, it is not strictly true. Differences can be found in proportions of length and width. The length of Small Caps is more than lowercase has, and width is more than uppercase has. Width of serifs and letter-spacing of Small Caps are similar. Harmonicity and optical accordance of elements to each other distinguish of "true" Small Caps. Text with scaled All Caps looks lighter and less spectacular.