Short answer: it might, but it depends.
To use an analogy, you could ask if changing your payment card PIN increases your card's security. The answer would be again that it depends:
- If you didn't use your card today, changing your PIN to a new
(equally complex) PIN doesn't increase security (unless you gave it
to someone or had your password database stolen).
- On the other hand,
if you were in a Third World country and used your card at 10
distrusted stores, then changing it could increase security, since one
of these stores could have spied on you.
So the long answer is that it depends on what your handheld did since its last reboot. If it stayed locked all the time and disconnected from networks, it is (almost) impossible that rebooting helps. Otherwise, it could help:
- If your handheld was connected to the Internet, there is a chance
that an OS flaw caused it to be infected (in particular if its OS is
- If you opened a legitimate app like a browser, there is a
risk that a vulnerability in it caused you to acquire malware.
- Most importantly, if you opened a malicious app, it could have activated malware.
In any of these cases, rebooting won't help if the malware installed itself permanently, but as explained by Esa Jokinen here, there is a significant chance that the malware is not permanent.
So the answer to your question is that yes, on average, rebooting reduces vulnerability.
Now, if you were to ask if the advice you refer to makes sense, you would need to consider the costs. The value of that advice for a given person depends on how much it uses its handheld, how much it knows about security, how secure the device is, and how sensitive their use is.
Rebooting has some costs:
- Environmental impact
- Lost utility from the device
- Potentially having to reopen certain apps
And, assuming the reboot is manual:
- Time spent launching the reboot
- Mental stress to think about rebooting
In my case, I will definitely not be rebooting daily. And unfortunately, I'm afraid the less someone knows about security, the more misleading this kind of advice can be (unless its value is properly explained).