The effect is an extra edge of surprise for the reader.
Explanation:
The exact rhyme is characterized by complete identity between the vowel ad all the following consonants in the two rhyme words. What really makes words rhyme is that the rhymes should have identical features. The slant rhyme describe cases in which the two words are not identical, but are featurally similar enough to suggest rhyme.
A slant rhyme is unpredictable, but it is phonologically unpredictable in a contextually predictable environment, giving it an extra edge of surprise for the reader.
