Share permissions only apply to the actual share point, security tab is where you lock it down so only those users/groups explicitly defined, either local or network, have access. Generally you should give everyone access on the share permissions, and then lock it down using security permissions.
You also have a lot more fined grained control over security permissions, such as creating special permissions to allow users to only write to folders, aka a drop box, or creating different permissions for varying subfolders and files in that share point.