Sine, cosine and tangent are just three different ratios of the same right triangle’s sides — knowing any one angle and any one side determines every other side and angle.
The mnemonic exists because all of trigonometry’s practical use — heights, distances, angles you can’t directly measure — reduces to picking the right one of these three ratios for what you already know.
It is for right triangles only. Given three sides with no right angle, or two sides and the angle between them, none of the three ratios applies and you need the sine or cosine rule. On a sphere even those change — the angles of a large triangle no longer sum to 180°.
Estimate with the rule, then check it against the calculator that models it properly.
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