In an article by Brad Morton, published in “How-to Geek”, the following points are made to support that suggestion:
This announcement shouldn’t be a cause for concern: no new specific threat has been identified, but fully rebooting your devices by turning them off and back on is part of good digital hygiene, and one of the 13 mobile device best practices being promoted by the NSA to help protect you from phishing, malicious apps, and other attacks.
Other mobile security best practices the NSA are suggesting you follow include:
Malware that records or interferes with your activity can also be stored in temporary files, which are usually cleared during a reboot. Your device’s performance may also be improved by regularly turning it off and on again, as buggy applications that may be using increasing amounts of memory will be forced to release those resources and restart.
To restart an Iphone, press the volume up or down and the side button, and slide the power button to power off.
For Android, press and hold the power button until you see the menu, then tap restart or reboot.
This same advice is relevant for tablets and Ipads as well !