Maybe I'm wrong but more than once it happened to me that some problems suddenly got fixed after a number of turn-on/turn-off cycles.
I also read somewhere that this was even recommended in some other OS's (Windows).
If a reboot fixes a problem, it's most likely faulty hardware, or a faulty driver for that hardware. Without any more details, it's hard to even guess what hardware or driver it could be.
I've certainly had single reboots help. Sometimes my hacking into running services requires a fresh start and that works for me. Kernel upgrades usually need a reboot to take too.
Multiple reboots only help if the system has to reboot in order to do something that in turn needs a reboot to take.
I have seen this first-hand with Windows but never with Ubuntu. Linux distributions are somewhat better at being able to push system-wide updates from a live session and load them all on the next boot.
If you find you need multiple power cycles to get something working, it's probably a sign of a deeper issue.
I want to ask which kind of problems can be fixed by reboot?
what I believe is reboot will help to kill unused applications that is run in the background. for example, some time you run an application and you close it this application somehow will still in the process doing nothing if you restart your OS this application will be killed, and that sometime happened with me even in Ubuntu, I close firefox but the process still work.
If that what you mean I agree otherwise I don't think so.
I hope that help