A Linux machine never needs to reboot, until it does.
Linux is known for its stability and reliability. Personally, I never rebooted my Linux machine until recently I realized that it is still running an old kernel version despite its being upgraded multiple times. To load the newer kernel, I had to reboot the machine.
Frankly speaking, the old kernel runs just fine. The only reason to reboot is to load a newer kernel. Sometimes it might matter, for example when there are security updates. To find out whether a reboot is needed, you can check for the versions of the running kernel and the kernel installed on the hard drive.
$ uname -r # version of running kernel 4.10.13-1-ARCH $ file /boot/vmlinuz-linux # kernel on the hard drive /boot/vmlinuz-linux: Linux kernel x86 boot executable bzImage, version 4.10.13-1-ARCH (builduser@tobias) #1 SMP PREEMPT Thu Apr 27 12:15:09 CEST 2017, RO-rootFS, swap_dev 0x4, Normal VGA
If the output of the two commands shows different versions, a reboot is suggested.