Hazrat Moulana Muhammad Ilyaas (rahmatullahi ‘alaih) once mentioned the following:
One of the purposes for nafl zikr being prescribed in sharee‘ah is so that the time when a person is not engaged in fulfilling his fardh obligations, he will be able to spend this time in engaging in nafl zikr, and save himself from getting involved in la ya’nee (futile and non-beneficial activities).
Shaitaan’s desire and effort is to destroy the noor (light) and progress that a person acquires through fulfilling the fardh obligations by involving him in futile activities. Hence, one of the purposes of engaging in nafl zikr is to safeguard oneself from this.
In essence, when a person has free time after fulfilling his fardh obligations, then he should endeavour to get engaged in carrying out nafl zikr and ‘ibaadaat (e.g. reciting tasbeeh, reciting the Quraan Majeed, performing nafl salaah, etc.) so that Shaitaan will not succeed in bringing harm to him by getting him engaged in futile activities (and sinful deeds).
(Furthermore, one special and important benefit of engaging in nafl zikr is that it [creates a noor in one’s heart which thereafter] enhances all the Deeni efforts one carries out. It also creates motivation in one’s heart to fulfil the commands of Allah Ta‘ala at all times and instils within him a strong desire to acquire the great rewards promised by Allah Ta‘ala for doing good deeds.)
(Malfoozaat Hazrat Moulana Muhammad Ilyaas [rahmatullahi ‘alaih] pg. 120-121)