LocalSend has been a godsend (pun intended) to me. I used Snapdrop/Sharedrop before, but it was always a coin toss if the transfer would work or not. I ended up switching to filedrop, but for some reason my transfer speeds were really low.
With LocalSend my issues have been all but resolved. I can send huge files between my pc and phone without fear of it disconnecting, and it works on my pc, old notebook, my dad's iPhone and my android phone. I really can't thank Tienisto (the creator) enough for what he built.
As @8orange8@lemm.ee said (https://lemmy.ml/comment/3459977), I believe they have different use cases. The TL;DR is: syncthing to have the same copy of a file across different devices, LocalSend to move files between devices directly.
On syncthing you have to upload the file to the synced folder and then download the file to your device, so like device A -> server -> device B.
Whereas on LocalSend you send the file directly between the devices, like device A -> device B.