Well, the button is big and red on purpose because it is supposed to delete data 
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This is a feature of a receive-only folder. Receive-only folders do not send any local changes to remote peers, but only receive changes from others. However, the folder still recognizes local changes. The big red “revert local changes” reverts any changes made locally, overriding your data set with the data present on remote peers.
In your screenshot you have 137 items (changed files, new files etc) which are only present on this machine, but not remotely. Since this is a receive-only folder, syncthing does not sync these changes. Pressing revert local changes will reset those 137 items to the state advertised by remote devices. This can cause deletions or reversions to old file versions - this depends on what the remote devices have.
“Local Additons” simply means that this receive-only folders has items present which were added locally, thus not synced to remote machines.