Defragging a normal Windows machine with a normal "spinning disk" hard disk drive rarely produces a noticeable speed improvement, certainly nothing like a lot of the 3rd party companies advertise, and even in edge cases where people are regularly creating and deleting extremely large (yet irregularly sized) files, you'll rarely notice any improvement from running it any more often than every 6 months or so. It's more of a psychological placebo benefit in most cases. SuperUser:Does Defragmenting really help?
Phones don't have spinning hard disks, they use a variety of flash memory, this has massively faster random read speeds than a disk that needs to rotate until the right piece of data is under the read head. Also the controller circuitry in flash memory arranges the data itself as the data is written, allowing it to "write-level" across all of the flash disk and avoid problem parts of memory. See these SuperUser questions on that topic for more info Do I need to run defrag on an SSD?, Is it bad to defragment a USB Flash Drive?
In addition from Honeycomb (Android 3.x) onwards, the Android OS by default doesn't give an attached PC full access to the internal file-system, instead of mounting as a USB drive it uses MTP to control how the PC can access the memory, and only the SD card is allowed to be mounted as a normal USB drive. As far as I'm aware there's no way for a PC to get enough access to the file-system to do a defrag over MTP.
Tl;dr the way that flash memory works means that a defrag actually does almost nothing, and may have a slight side-effect of slightly reducing the life-span of your phone's storage.