Establishing the correspondence
Comparing ABC with DEF requires specifying which vertices correspond. Opposite sides and included angles follow that assignment; changing the correspondence midway can invalidate an argument.
In I.4, equality of two sides and their included angle leads through superposition to equality of the remaining side and angles. The included-angle requirement cannot be replaced without examination by an arbitrary angle elsewhere.
References: [1]
Different criteria preserve different information
I.8 treats three corresponding equal sides. I.26 handles two corresponding angles and a side, distinguishing a side between the angles from one opposite an identified angle.
The sufficiency of these conditions does not make every collection of three measurements sufficient. Two sides with a nonincluded angle can admit alternatives, so a criterion must retain its stated restrictions.
Separating similarity from congruence
Equal corresponding angles establish proportional sides without requiring equal lengths. Doubling every side preserves angles but changes size; VI.4 treats the proportional relations of equiangular triangles.
Congruence is a modern organizing term for these results. Euclid did not explicitly define it in Book I, and equality of triangles can concern area. Historical terminology, individual conclusions and the modern congruence concept should therefore be distinguished.