Chapter 4 - Applications of the Derivative - 4.7 L'Hopital's Rule - 4.7 Exercises - Page 307: 2

Because they transform indeterminate forms into determinate forms that can be evaluated.

The method of straightaway substitution won't work if we get an indeterminate form. Then we use some special method to transform the limit such that if we substitute after this transformation we get a determinate form which can be evaluated.