One of the main motivations to encourage pair work in the English language classroom is to increase the opportunities for learners to use English in the class.
Example
The learners are answering comprehension questions in pairs after reading a text. This allows them to compare answers, and clarify problems together using English.
In the classroom
Teachers can evaluate the impact and effectiveness of pair work on their learners by using action research tools such as asking the learners how they feel about working like this or by actually participating in an activity in a pair and evaluating this experience afterwards.
Further links:
https://www.teachingenglish.org.uk/article/working-pairs-groups
https://www.teachingenglish.org.uk/article/group-work-v-whole-class-activities
https://www.teachingenglish.org.uk/article/increasing-student-interaction
https://www.teachingenglish.org.uk/article/teaching-mixed-ability-classes-1