I can't imagine it doing anything positive, just negative. Memory on phones is all flash-based. There is negligible if any fragmentation penalty. The read/write times are identical no matter where on the flash it is stored. Regular hard drives have fragmentation because of the spinning discs and moving heads to get from one part to another. Solid State Disks (flash-based) don't have a penalty and defragmenting them just wastes time and resources, as well as lowering the life expectancy of the flash chip due to all of read/write cycles.
ok - i will chime in with the anti-opinion... i CAN imagine it doing something positive, and if the defrag utility is written correctly, then it should not do anything negative... unfortunately, there are gonna be way-too-many possible ways to write the utility incorrectly - so ymmv... ie - _I_ will not plan to -try- any of the defrag utilities because there truly are many ways to do them wrong...
the reason that it -might- do something positive is that flash memory (depending upon details thatre specific to the flash chip which i have not researched here) can either be read/written in bytes or in -pages- (which for the sake of argument is some unit larger than a single byte, say 16 or 32 or 64 or... 1k?) the details are gonna depend upon the OS also... in general, the -best- that you might hope for is a -very-slight- increase in read-speed from some fragmentation of individual files (assuming they havent already been loaded up in ram to begin with) - and another -slight- increase in available space for saving further files...
however, i personally believe that if you are considering a defrag - that you should instead/first be looking into the oftentimes huge wastes of space that are your cache-files (particularly your browsers)... cleaning those out will reap much greater rewards for/in your filesystem than any defrag, imho... gl, h.
Really - +1 to HC's comment above. There's no moving parts - a bit at the far back end of the chip is electrically just as close as the bit at the front end of the chip.
A MicroSD chip is less than 1cm across. Electricity can cross that path in less than 30 picoseconds. That's 0.000,000,000,030 seconds Your computer took 10 times as long to add 2+2 together.
Defraggging is useful if you have a large platter physical hard drive.
And Flash has limited read / write cycles. Use up those cycles doing defrag and, well, you'll use them up. And your chip will quit.
Don't do it.