I have a windows device , if i have the folders in the local storage i can’t sunc with other devices ?( i don’t have the sync plugin ) .
if i can’t , can i put the folders in the cloud and sync with other devices ?
You can move your vault to a cloud folder like Google Drive or Dropbox and then access them from another computer as well. To Obsidian, it appears just like a normal path in the file system.
So what is the benefit of add-on services ( obsidian sync ) ??
The benefit is version history and that it will work natively with mobile (so that syncing is easy). If you only plan to use Obsidian on (multiple) desktop computers, then Google Drive or Dropbox etc. are good alternatives.
And you support the developers with a Sync subscription. 
you mean the sync is better , but it’s optional, right ?
No-one can force you to buy sync.
Do you mean that it is going to be optional for the apps?
No , i mean if i use the drive for sync , it is legal ?
Yes, it is legal. Obsidian works on a folder of Markdown files, these files are your files, Obsidian simply operates on them. You are free to sync it with whichever service you want.
Thank you for explaining
You‘re welcome.